Let's see. Jak. A guy who:

Could channel Blue, Red and Yellow Eco.

Discovered Light Eco and used it to stop Gol and Maia, Sages corrupted by Dark Eco, from unleashing it to destroy the world.

Travels to the future, that being Haven City and is immediately captured by Erol and Krimzon Guards and is separated from Daxter for two years, to be experimented on by Baron Praxis and pumped with loads of Dark Eco.

Thanks to Praxis's experiments, Jak can turn into Dark Jak, a super soldier with Dark Eco coursing through his veins and can use it to create shockwaves and cyclones and energy blasts and grow bigger in size.

Would I consider someone like that scary? After playing Jak and Daxter 1-3 on PS3 and getting their Platinum Trophies and am currently working towards finishing Jak 3 on PS Vita, I can confidently say yes. Dark Jak IS scary, because of what he's capable of.