Good vs Evil (Riddle)

Just a riddle I wanted to share with you. After 3 guesses from 3 different people, I will post the answer.
And I will not tell you if the guesses are right or not.

Here goes . . .

Two spies went into a bar, and each ordered a beer with ice. The bartender, however, was a bad guy. Seconds later, he received a call from his boss; "Poison the drinks in whatever way you can." And so the bartender, a very obedient old man, did.

The first spy immediately got a call from his boss; "You have an extremely urgent meeting. Drink up and report to the command center at once." The first spy did as he was told.
The second spy, who never received a call, drank his drink slowly, savoring every last drop.

The first spy survived. The second spy did not. Why?