So i modified a deathclaw a bit in the geck and when I get to him in the game he always runs. What is causing this and how can I fix it?

When you edit your creature, in the 'AI packages' tab there are different packages for patrols, sandboxs, dialog etc. In some of these packages, under the 'flags' tab there is an option you can place a check in for 'always run'. That is probably the culprit, uncheck that and it should walk and run normally.

The always run flag was already unchecked. any other ideas?

OK, how did you make the Deathclaw. Did you edit the base form and save it as the same form, or did you edit it then create a new form with a new formID?
What other modifications have you made to the form? Any scripts?
I guess I also need more info on the environment you set it in, if there are enemies around it is going to run trying to get at them if they are in agro range.
I really can't think of another reason it would be set to run all the time other than that or the run flag.

I edited it and created a new form. All I changed were the stats and inventory. There is nothing around it but a gate.

It's a common issue with custom creatures. I have no idea what causes this bug.
No matter the settings they flee as soon as you approach them instead of attacking you. It seems to appear only on some creatures while others react normally. There are some threads on the forum but obviously no one could provide a definitive answer.
Try to recreate your deathclaw, it may or may no work.
Do, you get any geck warning when you open the animation tab on your deathclaw ?

As Octarina says, this seems to be a regular problem. I had the same thing with a Mr. Gutsy. The only differences between my Gutsy and the standard game Gutsy were packages and factions, yet mine ran off as soon as I approached. A standard gutsy was fine. So I deleted and recreated my version and suddenly it was working properly. Absolutely no idea what the problem was, just one of the many peculiarities and annoyances of this game engine!