I'll slice you open like an envelope
This line is like a warning from a madman. It means you're going to get cut up and it's going to be messy.
He said, "I'll slice you open like an envelope," then took my phone and broke it.
In the hallway, she said, "I'll slice you open like an envelope," and then kicked my leg.
During the lunch break, he said, "I'll slice you open like an envelope," and then took my snacks.