MacLeod was raised to be a protector of his people, defender of the weak or of innocents. In immortality he's come to include most of mortal-kind and not a few immortals in the circle of those he must protect. Mortals are at a disadvantage with immortals, especially when unaware of immortality. Immortals for the most part cannot be brought to justice in the mortal system of law. So what to do when immortals break mortal laws, or cause harm to mortals?
MacLeod's actions throughout the series show that he consistently steps in to protect mortals from the predation of immortals, removing the threat the most effective way he knows - by challenging them to a fight to the death. But the question arises, when does this protection turn paternalistic? Does MacLeod see mortals as children who must be defended from harm at all costs, even when they put themselves in harm's way by choice? The argument that Byron led Mike and others to their deaths and therefore should be punished strikes me as condescending to these mortal adults. For they were adults. And adults make choices for themselves. Often they aren't the best choices. That's part of being an adult. And some choices can get you killed.
It's a cliche, but the old nag "would you jump off a bridge if so-and-so did" is all too appropriate. Mike followed Byron down a dangerous (ultimately lethal) path when he joined the party. The audience is not told whether Mike has ever used drugs before. He is a rock musician, so it wouldn't be totally out of this world if he had. Even if he had never touched drugs until he met Byron, he made the choice to do so, and to mix drugs and alcohol. Byron may have laid the heavy peer-pressure and manipulation on him, but Byron didn't hold Mike down and force drugs on him. And there is no evidence that Byron knowingly gave Mike an overdose, with the intention of killing Mike.
This type of incident is the impetus behind the "Just Say No" campaign in American schools in the 1980's.
Mike was not a child. He made the same kind of rash, impulsive choices young (especially male) adults make every day. Yes, he may have felt that he needed to play along with Byron's game in order to secure Byron's patronage in the music world. This is the kind of moral choice adults face, and must answer for. The difference between children and adults is that we hold adults accountable for their choices.
This does not excuse Byron's actions. He was possibly jealous of Mike's mortal passion, and just as rash as his "victims". He was a verbally passive-aggressive and used the promise of his influence over Mike's career as a childish tool of manipulation. He was clearly suicidally depressed, on a speeding train of self-destruction and not caring who he took with him. But I don't see the murderous intent that others argue was there. Yes, he wanted company in his risky flirtation with danger and death. He pressured, flattered, and wheedled people into joining him. But he never took their choices away. At any point, Mike could have chosen to walk away. He would have had to face the possibility of his career taking longer to ignite, or even face Byron's public scorn. Either choice had its risks.
As harsh as it is, as Methos said to Joe, "Sometimes the man isn't as strong as the music." This is an old story in the rock world. It's tragic, and people tend to seek someone to blame in a tragedy. People want a reason when the young and talented die. And most people don't want to accept when the realistic explanation is that the dead made a bad choice.
Mike fell for Byron's facade, disdained Joe's support and guidance. Mike buckled under Byron's pressure, and Mike died.
Byron chose not to heed his teacher's warning, chose to fight a man he probably knew was stronger and better than he was. And Byron died.