Drug issue is not an automatic disqualifier.
Being a knucklehead is not an automatic disqualifier.
Being a knucklehead with some past drug transgressions and rumors? That's a bad combination.
It's all about probabilities to me.
Probability that a player can develop into a quality NFL player (non-character issues).
Probability that a player will fail a drug test.
Probability that a players character will hold him back from developing as a player (coach-able, etc.).
Probably of injury (Injury history players).
I would only completely remove players that I just didn't want to have on the team. This is not a draft pick, but Greg Hardy was good on the field and didn't get suspended, but he was not coach-able (at least not by Garrett). This is the type of player that I would remove from the draft board if you know that the coaches will not want him on the team.
I would just de-rate drugs issue players or hot-heads as opposed to completely removing them from the board.
I thought Randy Gregory was a high probability of failing a drug test; whereas, Noah Spence was a much lower probability relative to Gregory because Spence had stayed clean for over a year, but Gregory failed at the very most important time (the combine).