I think because in the previous life (the one where sion still existed), he went against “God’s will” and all the terrible shit happened(all those wars, his brother dying, his only friend dying, most importantly Sion was captured and tortured)
In this life, he hopes that by following His will and do what an angel of judgement is supposed to do, Sion will finally have the peaceful life that he wanted after his sacrifice
(Correct me if I’m wrong other commenters)
that and also the fact that the stigmata and the sword of judgement coexisting at the same time only happened a few times in history and always ended in tragedy - so Pavel wants to erase himself to make sure nothing bad will ever happen to Sion and that he could live a peaceful life to the end (something he couldn’t give him in his previous life)
So what does Pavel hope to accomplish by completing his mission given by god? All this folklore plot stuff is kinda going all over my head nlg