1. It wasn't based on any particular urban legend. I noticed that other people were writing 1000 contest entries that were, so it occurred to me that an SCP could be responsible for urban legends.
2. Being able to kill people repeatedly might not be Keter-worthy, but factor in that it's extremely difficult to track due to A. You can't lock it in a box so you have to follow it around B. being virtually invisible, C. moving very quickly and erratically, and D. if you lose track of it, you probably have to wait until it makes the news (which is bad) to find out where it went. Plus, if you don't get some vehicles chasing it as soon as it leaves, it's pretty much gone until you can figure out where it went.
In short, it's not the lethality that makes it Keter, it's that plus the extreme difficulty in containing it. It's like a serial killer who you can't put in jail or execute; you have to follow him around and convince people not to go into his house, which has tons of money on the front porch. Also, he's invisible.