What alignment was Sam before this incident? Just a shopkeeper. No reason for him to be an evil antipaladin. He was just a commoner. Almost all commoners are basically neutral. I think LN is the most common commoner alignment - they follow the laws of their town and obey the authorities and take care of themselves and their families first.
Grief-stricken temporary insanity does not change Sam's alignment.
I agree with the others - there is no reason for a bullrush to kill people. Sounds to me like the GM said "OMG, this barbarian is irritating me, I'll punish him for wrecking my story - Rocks Fall! Sam's whole family DIES!"
In fact, though I wasn't there, I imagine the conversation went like this:
Barbarian player: A long line? I don't want to wait. I cut to the front.
GM: You can't.
Player: Why not?
GM: It's rude.
Player: My character doesn't care what city people think. He cuts in.
GM: There's no room. The people don't let you.
Player: I'm big and strong. I make room.
GM: There's too many people and they don't move out of your way to let you cut in.
Player: Fine. I use bullrush to push someone out of the way so I can take his place.
GM: *Gnashing his teeth and seething with frustration because his player refused to follow the rails* Fine! You killed Sam's new bride and newborn child! Happy now?So now it's done. Water under that weirdly unrealistic and exceptionally punitive bridge.
The OP said there was a "line up" to get into this shop. Surely that means that there were many witnesses to this senseless murder, right? The murder of a woman and child will be investigated by authorities in any town, even a Chaotic Evil village of goblins - even the most wretched hive of scum and villainy doesn't let outsiders walk into their community and slaughter innocents with impunity. The local law enforcement should be listing this vicious barbaric child-murderer as their #1 Most Wanted Villain and stopping at nothing to bring him to justice, regardless of whether Sam goes all...
You sir, have officially made my night (or morning, as it is 0100). This post made me laugh so hard I cried for many, many minutes on end! Additionally, my wife started busting up laughing too.
I have to throw in my .02c like everyone else and like Blake has so expertly done. Not sure how a Bull Rush pulls that off, but in my personal opinion, it sounds like a redact is in order (:
Even if you play it out, the Barbarian isn't going to get very far, as the local authorities would intervene within moments of this travesty and arrest this crazed, illiterate, NPC-murdering psychopath... thus, Sam's fate and subsequent "alignment change" is pretty inconsequential.