This is a game about killing things and taking their stuff. If you see an orc, you kill it. Why? Because it is an orc...and orcs are evil.
This is a falsehood perpetuated on and on (in most all editions) and there is nothing supporting it in the RAW.
First, for the larger majority of the people I know, this game is about WAY more than just killing things and taking their stuff. Maybe in a lite beer and pretzels game, but not a rich campaign experience. IMC, if you just kill things and take it's stuff, you will be hunted and/or incarcerated, or simply damned. This is where alignment comes into play, as vague as it is. Just killing things and taking it's stuff would place you firmly in the Chaotic Evil department.
If I can kill an orc (without knowing anything about it other than that it is an orc) and steal its property morally, then I can kill hundreds of orcs, also without reason.
If dragons are worse than orcs, then I can do the same.
This is where we disagree. According to RAW, an orc is "Often chaotic evil". According to the glossary, often assumes a plurality (40%-50%) of individuals. Killing an orc, without knowing anything about it other than that it is an orc, would be evil. In many campaigns, it could be considered murder.
Dragons being "Always" might mean that 98%+ are evil. But just killing them for no reason (or especially because the reason is vengeance, and the added layer of "and you will watch it happen to everyone you and your offspring have spawned in the past 801-1000 years") is still an evil act. The reason can't just be "I am pretty sure that he's still evil because the odds are in my favor". Even if 99.9% are evil, the sheer number of offspring over that may years, multiplied exponentially, will still result in many, many, many more "non-evil" deaths than just 3 (to make up for his wife and two kids).
I am reminded of the old Faberge Organics shampoo commercial. "If I told two friends, and they told two friends, and so on, and so on".