It's 100% a creaturescript of some sort.
I don't know how you aren't getting any errors in console though.
Start green texting everything you've added in creaturescripts.xml, and see if the creature dies.
If it does, then you know it's one of those scripts.
Start un-green texting the scripts in 'chunks' until the creature stops dying again.
Basically, just narrow down which script is causing the problem, then start figuring out why it might be erroring.
Add some prints into the problem script, so you can see how far it get's before erroring..
Once you find out where in the script it's erroring, then we can start troubleshooting why it might be stopping the monster from dying.