What happened ???

Well i dont know what got into marley today, on a walk in the park cheech of lead but i kept marley on as he was acting a bit strange,
There most have been 10 other dogs in the field and we had met most of them at different points but all of a sudden marley lunged at a little fluffy dog that was on the other side of the park and his flexi lead snapped!!!!! (i had the 25-40kg size because he is a strong little dog)
he took of over the field blanked all recall commands,just zoned out.
Luckily i managed to keep cheech under controlle (just, as he really wanted to help his brother)
a friend that was walking in the park kept cheech out of the way and i ran over to join marley.
I think he wanted to kill the other dog, it wasnt just a fight he was trying to kill her, i know shouldnt of but i reached in to pull him away he bit me and then i think he relised what he had done and he just stopped, looked at me and ran of to stand with cheech.

sorry for the long post i just dont understand why he would do that

Oh dear. You don't say what breed he is. But I can't think of anything to say about this apart from what I was told once, long ago, regarding my GSDs,that you sometimes 'never know what spooks them'.

I wouldn`t be trying to work out why if it were me. I`d be making sure it didn`t happen again and also working on socialisation.
What training have you done with him? Is he normally aggressive?

Maybe he just seem something furry, small and fast...then got carried away. Maybe a harness or head collar on a halti training lead would be a better option in future.

If he's been acting out of sorts all of a sudden then I would pop him to the vets for a thorough checkup to make sure it's not a health condition causing it.
As said above, I would now take extra precautions to prevent it happening again. You may never find out what caused it this time, and it may never happen again, but until you're sure he's safe to be off-lead again I would invest in a strong longline (less likely to snap or be dropped than a Flexi) and consider a muzzle. Remember to introduce a muzzle slowly so that he can get used to it.
http://www.youtube.com/watch?v=4H9om7v3nF0

he is fine today but i am not trusting him around other dogs for a while, other than cheech and my girlfreinds dogs
he had his halti back on for todays walk but didnt react at all even when cheech tryed to pick a fight with a dog whos breed will remain hiden

What breed is he?
I'm glad he is better today.

It might still be worth getting him checked out for hormone/thyroid issues, because it sounds like a sharp change in behaviour.

Is he old? If not, you could go back to basic socialisation again. It's hard to advise without knowing his breed and age though...

It must have been a shock.

he is a jack russell about 3 years old,

Maybe what you perceived as successful previous meetings with other dogs wasn't so? Maybe it's been an increasing spike of over arousal and concern . What were these, meetings like and where did your other aggro dog fit in on those occasions?

well with any girl dogs they just flirt and play
neutered dogs tend to be played with like you would expect with a bit of rough play and other intact dogs are all avoided other than one, (dogs choice not mine)

it was a bitch that he attacked