Does Venom negate deadly/critical strike?

No. Same goes for any other elemental attack. This misunderstanding originates from the fact that the DS/CS spark animation is being overridden by the cold/lighting/fire/poison animation.

This can be proven with the following test;

With Hero Editor, we create the following;  A 78-79 damage claw with 100% deadly strike.  An assassin with level 40 venom.  An opponent with 5000 life, 0% DR and 75% poison resist. The test is as follows; You hit the opponent and write down how many life he has lost. According to this number you can conclude if DS occured yes or no.

Physical Damage:	104 - 105	(17 - 18 PvP damage)
Poison Damage:	1075 - 1095	(45 - 46 PvP damage)
Minimum Life loss with DS/no venom:	17*2 = 34
Maximum Life loss with DS/no venom:	18*2 = 36
Minimum Life loss with DS/venom:	17*2 + 45 = 79
Maximum Life loss with DS/venom:	18*2 + 46 = 82

Hits without venom: 5000 -> 4966 34 4966 -> 4932 34 4932 -> 4896 36 4869 -> 4862 34 4862 -> 4826 36

Hits with venom: 5000 -> 4921 79 4921 -> 4841 80 4841 -> 4760 81 4760 -> 4680 80 4680 -> 4598 82

As you can see, the results live up to the rules we set earlier.