Passive starter kill is not a good idea.
All a thief has to do is jam a screwdriver in your ignition and tap the switch.
I would use a single relay and tie it in with the alarm.
Heres a diagram I drew up:
Starter wire = black/white. On ignition harness.
What you do is clip this wire. Your then left with 2 ends. The end that goes to the ignition switch gets wired to 87a & 85 on the relay. And the other end gets wired to 30 on the relay.
Whenever the alarm is armed, the car won't start.
P.S. You need a spdt (single pole double throw) relay.