NPCs killed by NPCs do not drop loot

[zeng-github01]

Issue description:
When an NPC kills another NPC, the defeated NPC does not drop any loot, even if the NPC is configured to drop items normally.
Loot drops work correctly when the player kills the NPC, but NPC‑vs‑NPC combat results in zero drops.
This suggests that the loot drop logic may only be triggered by player-caused deaths and is not firing when the killer is another NPC.

If crashed, link to crash report (use a site such as pastebin):

Versions:

• Minecraft: 1.7.10
• Forge:
• CustomNPC+: 1.10.3

Can it be reproduced with just CustomNPC+? If not, list the other mods required to reproduce the issue.