I have an idea for a Deathwatch campain:

Triston Holt, a Rouge trader of the Holt dysentrey is well known among the Jericho Reach. he is the one who discovered many of the Reachs now known worlds.

He has been tasked by Lord Millitant Tetrachus to take his ship, Sabre (used it from the rouge trader rulebook) into what is known as the deserters coil to bring the light of the emperor to this heretic region and a kill team is sent to help him take the fight to the foul heretic worlds

do you think this is a good idea for a campain of such?

Sounds like a good spring board for a long term campaign, although I don't know if the deathwatch would be given a mission like that. Of course, it's your campaign, so however you want it to go down. Just remember to give the players some decision making authority, and not put them on rails through the rogue trader.

Also, it's a rogue trader dynasty. Rogue trader dysentary is some type of space stomach virus.

Sounds good, but you must understand that the Rogue Trader and the Deathwatch are two separate organisations. Even Tetrarchus has to ask nicel to get the aid of the Deathwatch. In this case the Deathwatch could have their own agenda in sending the kill team. The Killteam would have much autonomy in this case.