is it ok to deceive criminals by pretending to be someone you're not?

i think it would be fun

not if it was a life and death situation

If people are committing crimes anyway and this is the only way to gather enough evidence for a conviction I suppose it is o.k., However that being said those on these types of operations should not use entrapment methods as it can be a legal minefield and also morally wrong.
Short answer yes it’s o.k. , Longer answer beware of entering a world of lies & violence as it can easily taint ALL who enter it.

Peace.

What if it could go wrong?

If I told you. I'd have to kill ye. Jk about the killing

It's great, criminals should be caught and put out of their misery ASAP.