In my experience, the best evil characters are practical. Moustache twirling cops are a parody made up by those who ascribe to the rather whimsical and impractical alignment of good- a laughable, mutable concept from this point of view. Money is needed to achieve certain lifestyle goals, and poisons are a way of quickly- and possibly painlessly, depending on the evil character's ethics- dispatching an enemy. It is just business. No less, but also no more.
An orc raped a human woman in a raid on a village. The village survived and soon you were born in a human village, and they accepted you more than most humans would. To them, a child is a child and it must be taken care of. You lived a good life, not quite in the lap of luxury but you were treated fairly. Your mother loved you, and you started growing smart, you were a curious young lad who saw truth and knowledge where others saw nothing.
But one day, the orcs came back. They heard of a child who looked like them in the village, and decided to punish the humans for taking one of their own captive. They burned the place to the ground and took you with them. Soon, however, they realized your skin wasn't as dark as theirs, and that you were not pure.
They treated you worse than they treat each other. When you weren't cleaning goo out of the chieftains toenails, you were being beaten by the others. However, you were still a keen observer and learned what you could.
The warriors went on a raid one day when you were 9. When they came back, they dropped a pile of books outside your hole. They didn't notice, and took the rest of the books to a roaring fire and threw them in.
Most of them were fairy tales which infuriated you. How could all these character have such great lives while you were stuck with these jerks! But one book caught your eye. It was a recipe book of sorts, only you didn't understand it.
When you were 13, you were properly initiated to the tribe. Even though they treated you like scum and hated you, they recognized your intelligence. By this time you had started to figure out some things in the book and were able to make acid flasks and alchemist fire. But some of the pages were more difficult to decipher.
By age 15, you had the freedom to come and go as you pleased. Something about the orcs made you stay despite despising them. You had more important things to worry about than residency anyways. Sometimes by yourself and sometimes with an escort of other young orcs, you went out hunting at night. Only you weren't hunting animals.
You had discovered enough about the book to know how to make some rudimentary poisons. The tips of all your weapons shone sickly yellow, and even your grazing hits did substantial damage. You were only discovering more as time went on, threatening young mage apprentices or alchemist apprentices you found until they would unveil their knowledge to you.
When you were 16, you were a full fledged alchemist. The fire that burned inside you for all those years, the same one that burnt down your village, the same one that urged you to hunt others was finally yours to harness. The bombs you could make, and potions you could brew proved useful.
Useful enough to finally do to those bastards what they did to you.
After disposing of your gracious hosts, anger lived on in you. You set up a lab where you make poisons and continue hunting people, for the thrill of new knowledge and the power you have learned to exert over them.
To justify working in a party, it makes it easy to hunt people or something I don't know. That was probably really bad and not what you are looking for, but hopefully it helps or gives you other ideas. Good luck!
First most villains don't see themselves as evil. They are either raised in a different moral system that the universe finds evil or they are trying to accomplish something that the ends justify the means. Many cases are slow gradual slipping. Enough compromises into the moral grey and your starting point begins looking suspiciously beyond your first endpoint. Love and Hate are both strong motivations to due terrible things for your own satisfaction. Some people develop a chip on their shoulder when terrible things happen to them. After a certain point they don't care about the world at large anymore, only about what they "deserve". Greed is a fun motivation, particularly when taken to far. It's plausible, easy for the gm to play with, allows for some wiggle room, and carries a built in character flaw to have some fun with. Poverty is a good reason to be greedy. Those who haven't had enough to live on can easily develop a fear of losing it once they are better off. As such they hoard as much money as possible so that they will never run out. The reverse could also be true. A now poor aristocrat could seek to regain his former lifestyle. Considering what money can buy in a world of magic, perhaps he is a do gooder hoping to fund a cause so noble that the ends justify the means. Money is an enabler, particularly when literal wishes are for sale on the open market. Greed is easily explained with any long term plan that requires resources or magic
Alchemy. This is a skillset. Just like swinging a sword or casting spells its just something you can learn for just about any reason. Perhaps you were raised by an alchemist father and/or mother. Perhaps you took an apprenticeship with a wise but perverted old mentor. Maybe you were simply curious about the world and wanted to uncover more about its nature. If teachers or parents discover a natural talent they typically push their child in that direction. Even if he didn't care for it, his ability for precise measurements, memorization of complex recipes, and exact labels attracted much attention. Just seeking to please his parents he learned alchemy. Alternatively a bored genius decided to learn it just to keep himself from being board. After all its more practical than chess. He could've been a slave or captive forced to work in a laboratory from the time he could walk. While never actually taught, he learned enough by observation to do a few tricks.
The focus on poisons in interesting though. Did you start by learning poisons or begin later in your career upon discovering a tome of forbidden lore? Perhaps poisons were the reason you started studying alchemy, so that you could harm your enemies in secret. Your master could have disavowed you upon discovering such dark practices leaving you with no choice but to continue just to support yourself. Maybe the poison started benign with pesticides and rat poison. Over time you became more and more obsessed with your power over life and death. Maybe you were being trained as a healer. In order to learn how to counteract particular poisons you had to first learn everything about said poisons.
Hope I gave a good idea or two. GLHF
Evil can be a lot of things, but often the scariest is simply in seeing others as more objects for your use/amusement/advancement rather than people in their own right.
When I think of an evil alchemist, I think of someone like Josef Mengele. He was, by all accounts, fairly nice in person to most of his acquaintances, he was a skilled doctor. And he experimented in obscene ways on prisoners, killed untold numbers, and caused untold pain and suffering. His subjects were, in the end, just 'things' to him, and they didn't matter.
More than enough evil for nightmares.
I'd put the focus on the character being interested in poisons. Wanting to learn everything about them. To develop new ones and improve their efficacy. You could even have an interest in antidotes as well as poison. Money, power are just means toward that goal, but not the goal in and of itself. The horror of this quest for knowledge is what you will do to acquire it, and how little you will care for what that costs others.