I'm glad it helps you vent some steam. It really is good physically and mentally. Although when I think of anger and parkour, it isn't like how you described. Let me see if I can create a fictional (although realistic, and loosely based off of real events) that demonstrates how I usually think of it. Hopefully someone else will see a part of themselves in it as well. Warning: as I got writing it turned out longer than I expected.
Go to the park so happy that everything I need to get done for the day is done. The sun is shining (even though the weather reports said rain). There hasn't been a better day for parkour. You make it to the park and to your surprise there's nobody there. It's a good thing too because in your area there's usually a bunch of toddlers and mothers at the park who don't see somebody having fun doing parkour, but somebody who is up to no good.
Just as you finish warming up you notice out of the corner of your eye a large black SUV pull up and out steps a heavier mother with bug-eyed sunglasses yelling at her two toddlers to hurry out of the car and to take their juice with them. You pretend not to notice and practice some lazy vaults, and general balancing. You notice that for some reason your balance isn't as good as it was a few days ago, you struggle to even stand on a railing.
15 minutes in more mothers and their kids show up. Now the park is swarming with at least 10 of them. One of them comes right up to you after landing a precision and says, "Woah, you can jump really well."
You start to respond with, "Well thanks kid, I practice a lot." but are cut off mid sentence by the child's mother yelling at her son to stay away from the weird big kid.
You continue to practice now being extra cautious to stay away from the packs of toddlers who are playing on your favorite areas. You continue to practice for a few more minutes but just can't get any good flow, it's just not feeling right today.
You decide to head off to a smaller, usually deserted park and get at least an hour of good training in. Halfway to the park a police cruiser pulls up next to you and the officer steps out.
"I've gotten a report of somebody that matches your description vandalizing public property." The officer says. "Could you step over here for a moment, and tell me what you've been doing?"
You explain that you have not been vandalizing property but practicing parkour. You explain parkour. The police officer asks, "Yeah I saw something like that on TV. Where you jump from one roof to another and stuff?" You have to explain that you have never, and don't plan on going onto roofs for this purpose.
He checks your record and sees no prior criminal offences. "Well I'll let you go, but I better not have any worried mothers calling in again."
You manage to make it to the park half an hour before sunset. Still a little shaken up about the police officer you shakily practice monkeys, then kongs, then dashes. But no matter how many vaults you do, you're just not feeling the parkour groove today. You deciede to throw in the towel for the day and head home.
You step off of the curb and
twist your ankle. How could your day have gone from so great to so bad? Sometimes parkour just makes you so angry.