Subliminals themselves cannot kill you. If you were to come across an evil subliminal maker with bad suggestions or affirmations on it telling you to harm yourself or commit suicide then it may be possible.
The subliminal itself cannot kill you it’s what you end up doing to yourself because of the bad affirmations recorded on the subliminal.
This is why it is very important to know what affirmations are recorded on the audio you are using.
If you feel bad or depressed after listening to the subliminal you should stop using it and figure out why.
And if you are wondering, subliminals can not kill brain cells or do actual damage to your brain, any more than listening to music would.