I guess this is a good way of looking at it, and in this case Hikozaru would be somewhat responsible for his trauma if he purposely stayed with Shinji knowing full well how bad things might turn out.
Hmm...I wouldn't say Chimchar was at all responsible for his own trauma - in a way, he was sort of like a child; Paul was most likely his only parental figure before Ash took him in (assuming that Paul's his first trainer), so Chimchar most likely wouldn't think much of Paul's treatment towards it; he probably just took it for what it was seeing as he had nothing to compare it to at that point. In that sense, I can see why some people would detest Paul for what he did to it - Chimchar was, afterall, an innocent creature who didn't know any better, and -as a result- willingly let Paul treat it the way he did, which in turn resulted in inflicted trauma for the poor monkey. It really did resemble a picture of an innocent animal getting tortured for ones' own gain, so I could understand why some fans would hate Paul to such extremes. Animal abuse is a real world issue after all, and I guess it's tempting to project that issue onto Paul and Chimchar's situation.
The way I see it though -and this is going to get me in a
lot of heat- is that Paul didn't know any better either. His impression of Pokemon and battling in general was just as skewed as Chimchar's impression of trainers in the beginning. All this time, he had only stuck with pokemon that are as ruthless and power-hungry as he is; he had absolutely no
inkling of an idea pertaining to dealing with pokemon that require a more empathetic approach, as those that do he'd most likely just release due to them being "weak", which, as I said, was easy for him to do as forming emotional attachments to his pokemon had never occurred to him before meeting Ash. This was due to what Brandon said to Reggie after his (Reggie's) lost to him (the key being
what Brandon said to him after the lost;
not the lost itself) - that he needed to become more powerful in order for him to beat him. Now, lots of people find it unrealistic that a single remark like that could drive Paul off the edge and cause him to become all power-obsessed, but think about it - let's say you haven't had any battling experience before, but you idolize someone (someone that's close to you, might I add) who's amazing at it. The only reason you idolized that person was because of his achievements and track record, as you didn't really have an idea as to what makes him so good. Ergo, you had no idea as to what goes into the making of a great battler, due to your inexperience in it. Then one day, that amazing track record of your idol of not ever having been beaten before was suddenly shattered by someone, said someone's only remark after his lost being pretty much "the only reason you've lost to me was because of your lack of power in comparison to mine." Wouldn't that leave a significant impact on you? You had absolutely no idea pertaining to what makes a great battler before, and suddenly a hint of what makes one was blasted in your face upon the devastating demolishment of the track record (iirc, it was 6-0!) of someone you looked up to so much for such a long time, said reason you looked up to him in the first place being due to that track record of him itself. That's ought to leave a burning impression on you in some way; your entire mindset about battling is bound to be built up upon that.
Typing this out, I actually realize why I had liked Paul's backstory so much in the first place. It's flimsy and stupid from a cursory glance, but yet, at its' roots, it actually makes sense and is believable.
EDIT: Also, upon typing that, I realize I've just unintentionally highlighted another parallel between Ash and Reggie besides being kind to their pokes: they both believed in unleashing the true power of their pokemon through kindness, but were met with formidable opponents that challenged this belief of theirs - Paul himself in Ash's case, and Brandon in Reggie's case. The difference between them was that Reggie gave up after it was challenged, whilst Ash persevered in it while integrating the best aspects of Pauls' training method (realistic assessment of his pokemon's abilities and potential) into his own. I just find it kinda poetic that one of them muddied Paul's perception of raising pokemon, while the other re-enlightened it.