Kels's middle ability is a triggered ability, not an activated ability. It doesn't allow you to sacrifice a permanent whenever you want; rather, you need some other way of sacrificing permanents, such as Kels's last ability.

If you sacrifice a permanent as part of a spell or activated ability's cost, Kels's middle ability will resolve before that spell or ability.

A legendary permanent that is put into a graveyard because of the "legend rule" isn't sacrificed.

If you sacrifice Kels, its middle ability triggers. It also triggers for any other creatures you sacrifice at the same time. You can sacrifice Kels to pay the cost of its own ability.

While resolving Kels's middle ability, you can't pay {U/B} more than once to draw more than one card.

You can activate Kels's last ability even if nothing's about to destroy it and even if it already has indestructible.