"In all fairness the forest god didn't belong in the city, but hellboy had to make the choice so to prevent a costly rampage."
umm lmfao, the forest god was and is older than every human city, he was probably pissed off at the city for being on top of where there used to be a forest a plenty, which the forest god could only imagine the genocide of his tree brethren, imagine how pissed you or anyone would be if your family was hacked up, used as lumber, or burned for warmth and then your home was turned into a f#cking walmart for a bunch of fat smelly idiots on rascals.

and then he is cruelly spurned by the massesafter saving them.

It wasn't a typical monster. The elf was trying to convince hellboy that he and it were both misunderstood creatures, and the last of their kind etc. In all fairness the forest god didn't belong in the city, but hellboy had to make the choice so to prevent a costly rampage.