I was so disappointed that they basically only alluded to his reasons in Suikoden 2, the expanded media makes him really... not sympathetic, but you can see his cause as one of a rampage of revenge and it's not really a surprise.
For those who didn't know;
Basically, when he was a child, the City State and Highland (his country) agree to peace after a long war that Highland basically came out the better for. The mayor of one of the City-States hires bandits that ambushes his carriage with mum and dad in it. The guards are killed, and daddy runs off (to get help). The bandits then proceed to gang rape his mum, whilst forcing him to watch. Eventually they get rescued, mum is pregnant and dies in childbirth. His rape-product sister grows up to be the spitting image of his mother, and she's always kind to him, which infuriates him because he hates what she is and has a constant reminder of who she looks like (yet he still bends the ear to her at times).
Hence, he's so consumed with bloodlust vs the City-State, detests his father, hates anything he views as cowardly (i.e. fucking everyone), and became that strong and maniacal so he could stand on his own two feet, and destroy the world of all who basically wronged him. And in a way it worked, he needed to be ambushed, shot with hundreds of arrows and he was still fucking going. He was a psycho, and he knew it and he never hid from that, he never begged for sympathy, he never brought up his past or anything, he just sat on that trauma and hatred and let it fester vs his father and the City-state. He's still human though, he shows surprising admiration and restraint to those with ambition and 'big cojones' and seems to have some empathy to those who suffered betrayal etc, but he just cannot comprehend why you wouldn't invest in the sweet joy of revenge and wanton slaughter, he's just fucked in the head in a lot of ways.
Not all of that is in the game though, you get some allusions to it, and you get the carriage being ambushed as backstory but they don't really get into the details.
Still, an iconic character, his whole 'die pig' routine is brilliant, his brute force is phenomenal and he is easily one of the best boss fights ever ~ you need a literal fucking army of ambushes and you still barely take him down and he goes out laughing his head off.