Personally, I think the guy could have benefited from another year in college, but maybe that wasn't an option.
He's got amazing speed for a guy of his size. However, his size appears to be one of the drawbacks. Too small to play on the D line and maybe too tall to play as a linebacker. I see him ultimately fitting in as a linebacker, but hopefully one that can be used to attack the QB. That would be best for him.
I don't know that anyone really knows what the "issues" were. Certainly the marijuana test did him no favors, but it seems others were finding other "issues." Whether that means not a strong enough work habit, disregard for coaches, etc. no one really knows.
Bottom line, he will still be paid handsomely and if he can perform on the field, he will be rewarded even more down the road.
Best of luck to him not only on the field but also in his life. Sometimes paying troubled guys a lot of money is the worst thing that one can do. I hope he handles it well.