This is probably very subjective but I think taming it would make it pretty much useless. Guts hasn't done anything really bad so far because of it. He wanted to kill his friends but in the end nothing really happened. I would like to see it actually going even further, not necessarily killing someone dear to him, just make Guts even more of a mad dog he used to be. Maybe it's me but I see Beast of Darkness as something separate, a being that feeds on Guts emotions and now he has more power than ever, it's just Guts who got also better at suspressing it. I wouldn't mind seeing Beast of Darkness being separated from Guts physically. That would be interesting to see how that would work but I doubt it will actually happen.It could, however, be tamed in a way that would make it more useful for Guts (Guts has already started this process).