I can offer a little insight into the mindset of someone who smears blood all over a cubicle wall, because I did it once (much to my horror now!) Warning - may be disturbing to some!
I was around 15, living in an extremely abusive/violent household (left me with permanent internal and brain damage), and had been kept isolated from the world (homeschooled, etc) until this particular time when I had been sent to some free computer training for school-age children.
The class was held in a local school, and all the other children there knew each other. I was also extremely dishevelled and smelly (had constant UTIs and no showers/clean clothes for weeks at a time), and made the mistake of telling the children there I hadn't had a bath for two weeks when they asked.
They taunted me, flicked snot at me, and spat chewed up tissue balls at me all through the computer training session, and the teacher seemed disgusted by me too (looking back I honestly can't blame him, I can't imagine how I must have smelt). In that moment, every single hope/dream I'd ever had about the 'outside world' being a better place when I grew up/left home came crashing down.
I felt a real rage come over me, almost literally like a pulsing red mist in front of my eyes, and I went to the bathroom and slashed myself with scissors (had already been cutting for a few years). I hated that place, and all the people there, and in my mind's eye saw it all burning down with them screaming inside (believe me, I know how horrendous that is now). I also had an incredible amount of pent-up rage from my home situation which just kind of fuelled the whole thing.
But in a mad sort of revenge-lust state, I settled for smearing my blood everywhere... every single surface I could find. There was no thought of spreading any disease or anything (I didn't even have this much awareness at the time), it was just that I was in enormous pain, and everyone seemed gleefully ready to aim another kick at me, and for once I just wanted "them" to feel shocked and disturbed, and to feel an unsettled ripple in their 'perfect world' (how it all seemed to my disturbed 15-year-old mind), and perhaps even to feel fear.
I can barely relate to that person I was any more, and remember all those years as if it were someone else, but it does all come crashing back when I hear stories of what other disturbed individuals do, because I was there once. I guess not everyone's motivation will be the same, but I doubt any of them come from a happy place.