Lots of blood, large cut, likelihood of it opening up repeatedly, hitting a vein, etc. are all good reasons to go to ER or to a doc who can see you immediately.
If it stopped bleeding that's good, but it's always a good idea to get it looked at.
I once cut myself with a S30V carbon steel Benchmade brand knife - which is to say, a very high quality, ridiculously scary-sharp knife that could do a lot of damage all too easily. I hit and completely severed a small vein because it went deep enough. I went to the ER immediately. It just wasn't worth taking a chance. Stitches aren't that painful IMO and are really easy to remove yourself if they give you a suture removal kit. If in doubt, have it looked at.
ETA: I just saw the pictures. That could have been worse. I'd still have it looked at though. Was the knife at all rusty? If so, you might want to ask your doctor about whether you should get a tetanus booster.