The Taylors were very quiet about the reason why Katie dropped Pete as her coach.
I wonder did she see him getting involved with a certain type of crowd and decide that she didn't need that distraction.
In terms of boxing "attracting" scum, I think it's more of a correlation thing than causation. Boxing as a sport is relatively cheap, and is popular in working-class areas as a place for troubled kids to get out their anger and learn some self-control. Your chances of "success" in boxing are also considerably better than something like soccer.
In middle and upper-class areas, fighting other people for sport is considered low-brow, so they pay to see poor people beat eachother up rather than get involved themselves.
Of course nothing is a magic bullet and a kid who starts boxing to try and turn his troubled life around, is probably just going to be a troubled kid who's good at boxing. And he knows other troubled kids, and probably has family who are similar. And so a hotbox of criminality builds up around the boxing club. Even if it's not the main trainers getting involved, there are people using and hanging around the club who are involved in criminal activity.
The description of a white van with UK reg plates suggest it was probably a traveller gang who carried out this shooting.
My sympathies with Katie Taylor who seems like a genuine person, and regardless of anything her Dad may or may not have done I'm sure she's climbing the walls right now.