@Grerutin Gotta love politics. Sometimes it's more politically expedient to pardon powerful people and have them work for you. Additionally, at least in this case, it seems that he didn't really do much of anything himself, other than stand guard at that tower to await her arrival; in other words, he allowed the revolution to occur and did not stop it, to pay a life debt to the person who saved his life, but otherwise did not work against the kingdom, which itself automatically contains a far lesser evil than if he had, for example, orchestrated all of the events.
@Doomer He spares his life because he is doing a job and as such, is obeying the orders of the one who hired him. As powerful as he and his company are, they could not survive against the entire world rising up to destroy them (at least, not yet; this isn't a spoiler, just that he has the potential to grow in power further and may one day be able to do so, I don't know), which is why he devised the idea of the mercenary company to begin with. That way, his people will have a way to constantly grow and become accepted by everyone else, instead of simply exterminated with extreme prejudice.
@Heinzart Exactly. Far too many people act as if forgiveness means that everything is back to how it is, when it isn't. It's merely the first step towards repairing the bridge you burnt.