I think its helpful to separate the the situation with Vincent and the baby from the fight with the NPC guards, because we really don't have enough information about the NPC guard situation.
The Vincent situation is an opportunity for some GREAT gaming though. Here's a guy who is ruthless, arrogant and has the law on his side--a very worthy opponent for your ruthless and arrogant PCs.
The mother asked the PCs not to let Vincent find out but he found out anyway and came to collect the child. Fine. Seems like the DM dropped the ball by letting Vincent find out, since that's where the big conflict was.
So then the players were in uncharted territory. The mother doesn't trust Vincent, apparently, but they don't know why. When Vincent starts acting suspicious, they fight and kill him. Again, I think the DM dropped the ball by not giving the players much to work with--there's nothing worse than a close-mouthed NPC in conflict with the PCs. That's basically giving them no other options than to fight or give.
If I'm the DM, here's what I do. Get the PCs to contact the mother again, via their Speak with Dead spell or other divinations. Go with Vincent being an evil, evil bastard. The child is extremely important. The King *must* know the truth about this child! For some reason that any DM worth his salt can sort out...
Unfortunately, the King has just declared the PCs outlaws, and all his men are gunning for them. Except for the other Arbiter, but he can't help them directly without risking the same fate.
Instant awesome.
As for the NPC guards that they murdered... there's not enough information there, but it does sound like they were acting out their adolescent fantasies and killing people who bothered them. Sounds like a good time to roll out some atonement.
The atonement is doing whatever the frack is necessary to protect the child and see to it that he does not fall into the wrong hands. This *must* cost the PCs everything. They should lose all their non-portable wealth. Their families are thrown in prison and killed. Anyone who helps them is an outlaw. Their old friends turn on them. And most importantly NONE OF THE MAGIC SHOPS WILL SELL TO THEM! ZOMGWTFBBQ!
