still haven't beaten this but i cant seem to google any info about it every post i get brings up people who just started the game and are still chasing benny ...

so once I complete this quest does my character die like Fallout 3 pre broken steel ? or can I just continue on with my wasteland adventures ? if game just ends and sends me to title screen so i can redo the whole assault again im reloading 3 or 4 saves back ...

I'm amazed that you didn't find anything else on this topic since there have been at least several dozen of these threads since I have been here.

No, there is no post-ending gameplay, the little pop-up you get before you go past the point of no return even indirectly says.
Yes, you should have played the DLC before hand and no, none of them give you post-Hoover Dam gameplay.

You should finish the DLC's before the Hoover dam assault,although there's a PC mod that lets you play the game after the ending.

.-. why did bethesda do this

how do i kill off the legion and stop their assassins ?

Obsidian, not Bethesda Understand that post-Hoover gameplay would do the game a dis-service and ultimately be an insult to the player. There are so many decisions our characters make and they have wide-reaching consequences. None of these would be felt if you could keep playing. Remember Broken Steel? It was a big F-U and basically rendered the Enclave as pointless.
If anything, Beth is to blame - they put on pressure with their time constraints. Even then, I'm not sure with more time Obsidian could pull it off. The ending slides are VERY comprehensive and to this day there is debate and speculation over the future of the Mojave. It's better this way, trust.

And those assassins are [censored] relentless, seriously. You're the Legions public enemy no.1 and they sure let you know it. You've simply got to be prepared with better weapons and hope you have the skill to use them. Don't worry, in time they'll become little more than a nuisance. If anything, with Jury Rigging, they're a godsend.