Out of the major canon races, which ones are you getting ready to rain bloody death upon? I reckon that the Eldar, Dark Eldar, Tau, Orks, Necrons and Tyrannids will be on the chopping block, as well as numerous smaller races which bedevil the Imperium which are less wide spread.
Here are some which I think would be fun to smash
1/ Eldar- Arrogant bastards. I get annoyed when I read about a battle where a small group of Eldar run circles around an Imperial army then dance through a webgate after their victory during which they did not suffer a single casualty. Eldar are the smart alec race which revel in their superiority over other races. A squad of Deathwatch in the right place could probably go a long way towards pushing them further to extinction
2- Tau- Bastart upstarts. How dare they think that their puny empire can stand up to the might of the Imperium of man. Their entire empire is about the size of a singe sector in Imperial space. They are expanding too fast and making a mockery of the Imperial army after debacles such as the loss of Taros (a book obviously writen by Tauphiles). It would be great to have a squad of Deathwatch marines running amok on one of their precious planets for months on end, ruining their ridiculous notion of a harmonious way.
Any other thoughts?...