runaway query

Nope. You'll use a fair bit of CPU, but there is a cap on how much you can use. If the query is causing disk usage you'll run out of io tokens and your linode will grind to a halt, but the host will not die.

The host won't die, but if you're pegging the CPU, you'll be competing for CPU with the other nodes on the host, theoretically degrading their performance slightly (essentially limiting how high they can burst when they need to). So it's good to still be considerate of your neighbors and not keep the CPU hammered all the time… but it certainly won't bring them down or anything. Since you're just one node they may not even really notice.