"credam" is not wrong, I think, but "credo" would be more literal.
Also, I'm not fluent in Latin but I don't see why the "enim" was added - to make it sound more idiomatic?
Shouldn't also a "te" be added to make it clear who does the killing? It's clear from the context, and maybe it's best to omit it, but as it's currently written the translation would mean "for I would trust you that (you'd) kill me".