Pride

Paladins are strange people. While many are happy pursuing their own goals in life, paladins abide by a code. Not just the letter of it, but the spirit of it as well. With most paths in life, people are at least subconsciously aware of what they will do and sacrifice to achieve their ends. "I would love to be Mayor more than anything, but I wouldn't kill anyone to do it" is just another way of saying "I prefer to not be Mayor than to be a murderer."
Paladins don't always have that. Some of them are missing the tiny voice that says "In carrying out your god's mission, there are some things that you personally should not or will not do." This is often praised as zeal, as young paladins are rewarded for carrying out their work without fear or selfishness. As they gain years and experience, most paladins find a comfortable balance where they carry out their duty with fervor and enthusiasm, but apply critical thinking to what they do, and restrain themselves when needed.
Jaq was not one of the good paladins. he was very good at carrying out his oath, and extremely good as acting as the bane of evil. He just wasn't good at cultivating the underlying humanity needed for those actions to mean something. If he'd been born in The Sands, he would have fit right in with the fascist regime there. Had he been raised in the daemonic realm of Abaddon, he might have been a force for terror. As it was, he was raised in the Valley, and became a force for Good and Law.
But without that little voice, things can begin to go wrong. While in jail in Skor he found himself in an odd predicament. Freeing himself would be a violation of his oath, but remaining here was preventing him from doing his important work. Freedom came suddenly, yanking him across the planes. There he found himself in a Devil's court. A choice was offered: He could fight here and now, to see how many devils he could take down before he was slain, or he could be transported to a volatile area where the Seven Heavens intersected with the Abyss, and do battle there, alongside the forces of Good and Law.
It's unknown if Jaq asked the obvious question of the devil: Why bring him here, instead of just sending him to the battlefield directly? It's possible that the devil just leaned back in its chair and said
"You need to make the choice. You've dedicated your life to destroying evil, and here are hundreds of devils on their home plane, where their deaths will be meaningful, where you will have made a difference in this war. Or you can go to the battlefield, and fight a righteous battle, angels against demons, good against evil. There you can fulfill your oath, and do what you've been trained to do"
Maybe Jaq paused for a long moment, before instructing the devil to send him to the battlefield, but we'll never know.
The wars of the planes are ones of offense. On their home plane, planar creatures risk permanent death, where their essence is returned to their homeland and their sense of self is lost. Gods can restore this essence and recreate the creature, but it an intensive process. If a creature dies while assaulting a foreign plane, its simply banished from that plane for a period of time that is proportional to the creature's power. For the common soldiers, this is usually from between a week and a year, which is an inconvenience at most.
Then there are the mortals who fight alongside them. Take this archer for instance; he can move back and forth across the battlefield without fearing that he is standing somewhere that will result in his permanent death, nor worry about being blocked from an area from which he is banished. He is weaker than most of the creatures he fights, but his arrows fly true and Muir's blessing causes them to defeat the defenses of anything he faces. Should he fall, one of the nearby archons will resurrect him and he will throw himself back into the fray.
Few mortals would choose this life, and fewer still are powerful enough to withstand it. The constant tearing of soul from body and back again can take a toll on even the most hearty of mortal creatures. But this archer revels in it, loosing arrow after arrow, cheering whenever his foe falls. If one was watching him, they might almost think he was having the time of his life.