I buy games from G2A also. Hope i'm not doing anything illegal by buying cheap stuff sometimes from G2A?
The competition will tell you they're shady but I'm not convinced. You're not breaking the law, I'm pretty sure of that. No moreso than randomly buying stuff off Amazon or any other online marketplace. I'd take most of the supposed legal complaints with a huge pinch of salt.
For me, the pecking order goes: GOG/STEAM, GMG/Humble etc., Kinguin/G2A... the rest (apart from EGS).
Generally I'll go with the first four, because I want devs to get a big cut and because I don't want to be dealing with weird activation routes or blocked STEAM features, but if prices are stupid, I'll look elsewhere. No actual experience with CDKeys, only because their name sounds kind of backwater, but nothing particularly against them over G2A etc. I've bought from most of the larger sites at some point and never had any issues.
Can my games from G2A get rovked even if i activated them on Steam?