I don't think they neccessarily ruined Alexander. True, they aged him about 10 years, which is a common soap opera trope, but that can be explained through fanwank: Klingons mature more rapidly than humans, and Alexander is mostly Klingon (they did the same thing with Naomi Wildman on VOY and explained her rapid aging as inheritance from her Katarian father, because Katarian children conveniently mature faster than human ones).
And yes, when we saw Alexander as a child on TNG he wanted nothing to do with Klingon culture and being a warrior, but given his father pretty much ignored him after TNG, maybe he subconsciously felt he was being punished by daddy for not embracing Klingon values, and so did a 180. Unfortunately, since he didn't keep up with the little training Worf gave him in his childhood he was woefully underprepared for combat. At least he got his father's attention, which he was understandably desperate for. Also, as he got older I'm sure he realized how very 'not human' he was and maybe regretted turning away from Klingon culture, because he probably didn't fit in very well on Earth.
Frankly I'm glad they brought Alexander on, even if it could have been handled a little bit differently. It's better than just forgetting/ignoring the fact that Worf had a son, which they'd already done for long enough.