Odd scary customer

[Lirange]

So today at work a customer walks in and I'm the one who has to take his order. The first thing he says is "great I get you two times. You can't understand me." I tried to take this mans order before but he has a speech problem so I couldn't understand him and he got frustrated and stormed out. So I try to ask him multiple times if he'll like someone else to take his order but he won't listen to me and keeps on rambling on about me...i think. So whatever, I decide I'll just takes his order. Thankfully I can understand what he says. He said he wanted nachos with no onion or peppers. I know what he wants but I inform him anyways that our regularnachos dont come with onions. He gets frustrated and repeats hisorder so I decided to let it be. I tell him his total and he throws his card down. I said thanks like I always do after someone gives me their cards and he mocks my thanks. I lay the card down(i put it down on the register,i usually give then the card back along with their receipts when it gets printed) so he reaches over and grabs ithimself. The merchant copy prints so I put it in the register and print the customer copy though I just walk away as it prints because I know he'll just tear it out himself, which he does. And he starts talking about nonsense we can't understand towards my coworkers and I just try to hold back my laughter. I dont know if I should feel sorry for him or not.i was never rude too him butt he still acted like such an arse.also he also has muscle problems or something because he makes weird twitches or vibration with his arms so I was afraid he was going to lash out and hir me.