JellyElite is committed to the bit. Wearing the Symbol of Avarice (the treasure chest head), he crouches like a preying insect over some tasty loot. Invading players are faced with a choice: go for the treasure, attack the mimic, or pacify Jelly as you would a real mimic. Two of those are bad options.
Because few players (me among them) knew you can sedate a mimic by hitting it with an Undead Hunter Charm, bloodshed ensues. Admittedly, Jelly has an advantage over NPC mimics by virtue of two mimic friends rushing in to devour the invader. Let's call it artistic licence.