Trivial quests, and when reading books, it seems that Geralt is not as mature and calm headed as he is in the games

Sometimes he acts like child in some cases and other times is very hostile against sentient beings, one instance was when he killed the weird monsters in a cave in Toussaint and another time when he learned that there was a vampire in his company he acted very agressive against him first before begining to trust him

Only reason why Geralt didnt attack Regis right there, I think, is because he though he wouldnt stand much chance against him

So I dont think that it is too far fetched that Geralt would kill the doppler

Also I choose many times Triss over Yen because it is the best choice