https://www.bbc.co.uk/news/articles/c1dzq41n4l9o
> Giving evidence earlier, he said the group's only intention was to "break in, cause as much damage to the factory as possible, destroy weapons and prevent the factory from reopening".
I count "causing as much damage as possible" to be violent.
While I think graffiti taggers "damage property" but are non-violent. But in many places, rival gangs blow up/set alight/demolish their rivals' homes/businesses/vehicles, etc. That counts as pretty strong violence to me, even if no people are injured.
Anyway, talking of people being injured, watch a member of Palestine Action (Samuel Corner, 23, Oxford University graduate) drive a sledgehammer into a police seargent while she's trying to arrest his comrade:
https://www.bbc.co.uk/news/articles/c4g54g1r15eo
Full video, sledgehammer attack at 3m05s to 3m10s: https://www.youtube.com/watch?v=l6P7p_5D4hw
I'd designate them as a terrorist group for destroying factories, not so much for spraypainting planes. But I'd still support your right to say you support them, even though I'd disagree.
> I count "causing as much damage as possible" to be violent.
That is just not what the word violent means (unless used figuratively but I don't think that's what you mean). It means hurting, or attempting to hurt, a person (or maybe an animal). Setting fire or blowing up a home which might have people still in it is certainly violent, but destroying property for the sake or property destruction is not.
Of course, deliberately attacking someone with a sledgehammer certainly is.