Wait, why didn't he just copycat plunder before doing the split up and homicide. The fuck sorta calculations are he making.
(yeah, I'm aware that this is a half-decade old post - but nonetheless...)
Some of the earlier chapters imply that he didn't actually know who had the Plunder skill.
You know when Haruka comes across the jock group in chapters 13 and 14? They tell him that they're keeping their distance from everyone else because
someone has the Plunder skill, and they don't know who.
And then later into ch.14, class prez comes forward and tells Haruka that she's the one with Plunder, and that she kept this a secret from everyone except the otaku group.
Tanaka was already running with the delinquents at that point, so he wouldn't have known back then.
Tanaka probably figured it out on his own later on through deduction. "Hmm, I haven't seen anyone use Plunder yet, and I've seen pretty much everyone use their skills - except the class prez..."
Why did Tanaka have two skills? Why did he even chose Copycat, when he just could have just taken Plunder from the God's board?
Tanaka having two skills, that one is easy;
It's not "one skill per person" in this series - it's a point-buy system. Each isekai'd kid got 50 points, and each skill or item could be bought for an
x amount of points. The first chapter explains this (albeit briefly).
So it's perfectly possible for these kids to buy more than one skill. As long as the total doesn't cost more than 50 points.
I suspect that Plunder is also a really expensive skill - something like 40 or 50 points - and that it may have drawbacks like automatically killing your target, or forcing you to kill your target before you can actually use it.
Copycat was probably much cheaper, allowing Tanaka to take a second skill and thus become more flexible.
Or alternatively, maybe the class prez just got there first and ended up picking Plunder for whatever reason.