Van is so defensive.
"I wouldn't be able to stand it if he asked for my forgiveness and apologized, especially with that dhampir girl he saved. That's the same as saying he's a saint and I'm the evil one for not forgiving him! Foul! Even if he meant good, I won't forgive him!!"
Van is the only one saying that he would be evil for not forgiving Heinz.
Not forgiving Heinz isn't evil, but killing Heinz and even destroying his soul is a much worse fate than what Heinz did; that being knocking out Darcia and handing her off to the requestors, ignorant to the fact that he would doom Darcia to be tortured and burned at the stake out of his own naivety.
Basically, because Heinz took a guild request and knocked out Darcia, Van would kill him and shatter his soul.
Even if Heinz prevented another dhampir child from suffering; one that isn't blessed with his toughened, 30-something year old mind and soul, nor his special magic or his endless mana, even if Van has a clear path to revive Darcia using her soul, and can keep her soul alive indefinitely... Van still only chooses to consider the wrong done to him, deciding to kill Heinz eventually.
I mean, if Van kills Heinz after the dhampir girl had stayed with Heinz for years, he's basically doing the same thing that Van blames Heinz for; killing that girl's parent.
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Van's revenge is clearly wrong, but he chooses to ignore all of these reasons that I'd wager that he also thought of, and holds onto his hatred towards an objectively good, better person.
If Van kills Heinz, he should at least not delude himself that he's justified.