From a marketing perspective it seems like the easiest way to leverage yourself into an audience to do all the viral-ness for you. He is like a parasite and PewDiePie, since people debatably say he is alt-right (I don't know anything about him but the more debatable this is the better) and popular, he makes an excellent parasite host.

Pick a popular figure that people claim is [adjacently controversial] in some way, now that popular figure has to talk about you to disavow your crazyness etc, but if they do, your plan has already worked: You've just hijacked his audience into hearing about you. And if the popular figure stays silent to stop this, other people who hate the pop figure will trumpet your name even louder and farther in trying to get the popular figure to say something.

The Verge article linked above has a compelling reason - it is a meme which will resonate in the media coverage, and it also forces PDP to address it publicly, which raises the crime's profile yet further.

Probably calculated to cause even more coverage, as strange as that may seem. Or maybe for the same reason he put the "navy seal copypasta" in his manifesto.

PewDiePie panders to the "alt-right":

https://www.vox.com/2018/12/13/18136253/pewdiepie-vs-tseries...

https://www.theverge.com/2018/12/10/18134748/pewdiepie-er-pe...

https://www.thisisinsider.com/pewdiepie-response-nazi-youtub...

There's a long tradition of people yelling slogans before going to war, or assassins yelling the slogan of their cause or religion before murdering. Evidently pewdiepie is someone he was willing to murder for.