When playing Talisman as the assassin, can I assassinate creatures I encounter following the draw of a card?
According to the official errata (linked here for reference), yes, you can assassinate creatures you encounter on cards you draw.

Q1: Can the Assassin assassinate any creature, regardless of whether or not it is drawn from the Adventure deck or already faceup on the space when he encounters it?
A: Yes.

Essentially you can always assassinate a creature you do battle with (i.e. has a strength value that you are choosing to engage in battle), unless the rules on some other card specify otherwise. Regarding pit fiends or the werewolf (or other board enemies like the sentinel, mutant, ice beasts, etc.), that seems like a separate question so I'd post that separately.