Well, some "monsters" are really "bad guy" humanoids, so if you don't believe in "Always Chaotic Evil" then it makes sense that goblins, kobolds, lizardmen, etc. might find common cause with humans, elves, etc. (Finding lodgings and other "civilized" comforts might be a little trickier, if NPCs still believe in "Always Chaotic Evil".) There's also the excuse of Drizzle Drow-Angsty, or whatever his name is: after a childhood filled with indoctrination, your character rebelled and realized Evil is Bad.
On the other hand, the excuse for Mind Flayers, vampires, etc. is a little more strained. "Hi, I'm Kshloork the Ilithid. As long as you don't try to kill me, I won't eat your delicious brain. You have my word."
"On two occasions I have been asked [by members of Parliament], 'Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?' I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question."
- Charles Babbage (1791 - 1871)