do you always change the verb following one of these forms top past tense?
ex: i should've killed him when i had the chance.
"Should've killed" = "Should have killed".
If you weren't intending to use the present perfect, you wouldn't be writing "should have" in the first place.
Note, the past tense is not always the same as the past participle. "Should have gone", not "Should have went".
In the case of "kill", they are the same.