Can you remarry if your spouse is dead?
Solution 1:
Yes you can: source from the Elder scrolls wikia
The above suggests you need to use some console commands too, but most people are reporting that you just need to put her down and remarry.
Update: I just found this on UESP
Murdering your spouse may result in not being able to remarry (confirmed on PC, Xbox 360 and PS3)
So it seems very hit and miss.
Solution 2:
No you can not remarry, to my knowledge, if you kill your wife in-game.