Yes, you could do that but then your node would be kicked off the Tor network (because you'd need to do it indiscriminately since you don't know who the user is you're trying to target). In addition, relays are load-balanced based on trustworthiness and bandwidth so in order to attack a significant portion of users you'd need to be running a large and trusted node (which would be hard to do if you're just doing this to attack people).
I wasn't aware that Tor tested services and had a trustworthiness score but an attack like that could still be quite useful for certain purposes and possibly stay well hidden. If you set something up that only did it for Google IP blocks for example it might go undetected. If you actually got shut down you could refine it by only targeting a small percentage of those users. There would be some rate of account collection, however small.
