Poison him with a Blowjob

You take a tube of lipstick and apply it to your lips until they are crimson. Kiss the guard’s neck, “Would a big sting man like you like some help?”

You place your hand on the bulge between his legs. The throbbing member easily extracted from the armor. You get down onto your knees and place your crimson lips around his cock. He’s not fighting too wrapped up in the moment to realize what’s happening.

He puts his back against the wall, “Me, lady... keep going!” He says excitedly.

Your lips go up and down his rockhard cock until you can feel the pleasure building up to the brim. You lick the tip and sweet cum fills your mouth as you drink it all up. The guard shudders in exquisite pleasure. He collapses to the ground, “That was the best head I’ve...”

And then he is quiet. The lipstick was specially made. It would only kill someone if you give them head. You wipe it off to avoid any further complications, just in case. And enter the vault.