Can you activate a gear's ability that costs 'Kill this:' at any time during your turn, or does it require an external prompt?

Ruling: You can activate a gear's ability that costs "Kill this:" on your turn when there isn't a chain, without needing an external prompt.

Nuances:

• Normally you cannot kill a gear by choice unless an effect specifically allows it
• If the ability required an external factor to trigger it, it would be worded as "if/when killed, <then effect>" instead of "Kill this:"