Can you target a unit in a base with a kill spell?

Yes, you can target a unit in a base with a "kill" spell, provided the spell does not specifically restrict its target to a "battlefield."

According to RiftJudge FAQ #7344, if a card does not explicitly specify "battlefield" as a targeting restriction, it can target units in a base. If a spell states "Kill a unit," it can target any unit on the board, including those in a base.

However, if the spell specifically says "Kill a unit at a battlefield," you cannot target a unit in a base. Additionally, as per Rule 359.3.e.9 and its example, if a spell targets a unit "at a battlefield" and that unit is moved to a base before the spell resolves, the target becomes invalid and that part of the spell's instruction will not be executed.