How I see it Chekov was assigned to the science station after TWK and he command the position at the start TSFS so he would have access to the codes for the auto destruct.
Oops, true. So it's a trait shared by Janeway and Sisko. And, if we count extended editions, by an older and wiser Kirk as well, because in ST:TMP DE, he finally agrees to Scotty's rigging of the ship to blow... Timo Saloniemi