when i pay 2 for the zhonias in base and a friendly units die y must use zhonias of i can kill the unit instead?

No — you can't choose to just kill the unit. Once Zhonya's Hourglass is face-up on the board, its text ("If a friendly unit would die, kill this instead. Heal that unit, exhaust it, and recall it") is a passive replacement effect with no "may," so it applies automatically the moment a friendly unit would die. The only choice you get is which unit to save when several would die at the same time (Rule 373).

The 2 Energy you paid was just the cost to put it into play — there's no further cost or "activation" to use the effect, and no option to skip it. If you have some other replacement effect on the same unit that prevents the death outright, you order them (Rule 372) and Zhonya's can go unused, but on its own you can't decline it (FAQ #331).