The question would be, how would integra's death affect the seals?
I know that he is bound to the (van) Hellsings, but I'd think Abraham would have some sort of safety built onto the things, nobody is above fate, and all lines must come to an end at one time or another...
I agree with Dhampir, for all I know of the walter character, he would have been eager to turn the round table onto mincemeat, literally. It would need a very particular circumstance for Walter to betray, speciallly when he is not a tactical idiot. (ai, if it is for protecting integra, once alucard is dust, Integra is no longer a valued piece and becomes disposable...)
Of course, our redophillic friend might be confused, or mistaken (still, I find it difficult for any force to take hn unaware)