He's been trying to participate in opposition politics in South Sudan and imprisoned for it, likely because he's a real threat to power. I think it's too much to label him a crook, he's just fighting corruption by actual crooks with tactics others will obviously disagree with. He's definitely a radical activist.
I'm not denying that South Sudan's leadership aren't crooks, but the only way you can climb up in these kinds of environments is to be a crook yourself - reformists do not survive in political environments where violence and criminalization is normalized. And this is part of that naïveté I mentioned earlier.
And if a coup were to happen, would stuff even get any better? Countries run on institutions, and a coup by default undermine institutions by normalizing violence as a short circuit to consensus building. This weak institution building due to the power of the gun is what lead both Argentina and Pakistan to stagnate despite being economic darlings in the early and mid 20th century respectively.
And more critically - we in the US do not want to normalize private citizens fundraising armed movements en masse in the 21st century. This is a bad precedent.
Finally, the Wikipedia article has clearly had some PR panache attached to it based on the edit history, and it's silence about Ajak's case despite having been going on for over a year now.
https://en.m.wikipedia.org/wiki/Peter_Biar_Ajak