Maybe obvious, but uh: don’t fucking do this! I get it, I'm an older brother, I would wanna go ballistic too. But the fact that these guys skipped over "notify the police" and went straight to "seek out in-person confrontation" is insane! Stalkers are not people in their right mind. Staking out and planning to “citizen’s arrest”(???) someone who has repeatedly visited late at night to threaten and harass is NOT SAFE…
Unless you have independently evidence (that can be independently verified) of a better than misdemeanor crime police almost certainly aren't gonna do squat other than put you on a list of "people with motive" to check up on when the dude you're after gets shwacked by someone else he's wronged.