I'll slice you open like an envelope
This is like the worst insult you can get. It means you're about to be gutted and you're not even cool enough to deserve it.
He said, "I'll slice you open like an envelope," then pulled out a knife and started chasing me.
During the fight, she said, "I'll slice you open like an envelope," and then tripped me.
He said, "I'll slice you open like an envelope," and then sat on my face.