Tanaka has "mimic," it allows him to copy a temporary / weaker version of other skills. He copied Plunder from the Class President, and then manipulated the class into factions and played them against each other. Once he got strong enough, he used Plunder (Mimic) to kill some classmates to steal their abilities. That's the downside to Plunder, you have to kill the target to take their skills. He specifically has killed all the delinquents and others, a total of 12 people.
A stated thing is that everyone's comfortable with the Class President (or Haruka) having Plunder, because everyone knows they would never use it against them. Also, Haruka keeps wondering if the Class President stole the "Extreme Libido" and "Unbelievable Sexual Stamina" skills from Goblins and Orcs, which is funny.
He left the Jocks alive intentionally to follow them to the Class President, but he didn't realize they had met with Haruka, so instead they went to Haruka for help and he healed them and made them leave, leaving him alone despite him being "weak," only a level 9. (Now 11.) This was the first mistake Tanaka made, expecting them all to team up against him.
The other mistake he made is thinking the "useless" skills aren't still "cheat" skills -- for example, Haruka has "Hikikomori," which makes him super powerful when defending his home. Guess where Tanaka just attacked him?
His final mistake was overrelying on game mechanics and numbers (math nerd, remember?) -- so he looks at Haruka's level 11 and thinks there's no way statistically a Level 40-someodd Tanaka can lose. But Haruka has literally infinite luck and several skills that screw with the leveling and power system (NEET, Loner, Jack of All Trades, etc), so he has no idea what's coming.
