I believe it is a condition. A social condition, not a medical one!
The meaning of Incel is fairly broad, but adheres to one general description. However even that can be inconsistent or inaccurate. The cases of Incels need to be handled on an individual basis, rather than as a collective. Generalizing a very loosely defined group as terrorists isn't really a good idea, and it will continue to stigmatize this specific group. There are indeed some good men who would be considered Incels because they really are involuntarily celibate, due to various factors such as appearance, wealth, social skills, etc. not because they're a raving lunatic. I believe the solution lies in tackling the root of the problem, which would be "toxic masculinity" as it is known in one format.
Although I seriously believe that phrase needs to change, as it has been hijacked by various people using it to demean or otherwise insult men.
There's good intent in that last line, but you're just looking at the 'mental illness problem', where they keep inventing new terms because the old ones were made offensive. The "r" word used to be professional and technical, for example.
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"A guy talking into the camera about politics? My favorite." - Phantom_Nook Guide, it's Guide, it's that Guide