Isran keeps trying to kill me

Solution 1:

From A New Order

Return to Skyrim - I had to run the whole way.
Wait for 24 hours (I went back to Riften), and try again, they were peaceful this time.

Solution 2:

I encountered this problem too. I came to Dawnguard and found Isran and his company fighting vampires. I shot at them with my arrow, but I misfired on one shot and it hit one of Isran's company. Because of this, they attacked me and would not stop. I went back to Riften and found I had a bounty on my head because of it. I believe this is the cause of them attacking you, but for good measure I also waited 24 hours, as suggested above, came back and then I could talk to them.