Van understands that the other Reincarnators coulldn't have known the Undead was him. That, plus since undeads in Origin tend to be mindless and the bad first impression that he made (he had just slaughtered a bunch of people), it's normal that they murdered him.
So he forgave them. He will still kill them if they try to urt him or someone he is closed to, but he will also help them if they become his allies.
As for those who killed his mom, he can't forgive them. The last time he tried, he power leveled his [Bad Status Resistance] and [Limi Break] skills, while also getting several ulcers and puking blood. So yeah, forgiving them would actually kill him.
But he has other goals in life than murdering them all, and he will only do it if there's a possibility to kill them without putting himself, or others, in mortal danger.
He actually ignored one of his target several times because the conditions were not good.