Killing a former tyrant against mortals is hardly "their" way. How is that about them? Just because they're adamant about justice doesn't necessarily mean they're merely self-interested.
Paarth has two things going against him, that some like to ignore. He's not only a dragon, which already makes him a threat, but his past makes him a threat.
Note, I'm just saying a "threat". If people can at least acknowledge that, and still choose him, then that's cool. But it's kind of annoying when people call him "cute" or "awesome", and two mere humans are the scourge of the earth or something. They did nothing as bad as Paarthurnax. Not yet at least.
Things can be cute and still be dangerous. Why do the two have to be independent? I admit he is a threat, but I will temper that by saying he has more charisma in his little claw then all the blades combined.
Another thing people tend to forget about Dragons in these games is that they are not hatched. Dragons are timeless creatures created by Akatosh. He wasn't born innocent then turned evil. He, like every other dragon, was created that way and then turned against it. The blades make a big to do about punishing Paarth for the crimes he once committed and say that even though he is now good, we should not pardon his past offenses. Normally this is true but in this case it's utter B.S. since Paarth initially had no choice in the matter. But lets put both sides likability aside for a moment and just look at just what they can offer the world in the long run...
The Blades are no longer an international military organization. They are two old veterans on the run. The purpose of the Blades is primarily to serve the Dragonborn, so they could over a great opportunity for the Dovakiin to begin to build up a force to resist the Thalmor. But, as previous posters have pointed out they have let their duties slide and now only seek to control the Dovakiin for their own purpose.
Paarth on the other hand is a significant threat by his own admission, but also is the greatest hope for stopping the Dragons short of genocide. If he is left alive he will go forth to attempt to teach his methods of controlling their lust for power. A benefit for the world that the blades cannot match.