@matigo Rather than allowing your "dark thoughts" to force you to come up with justifications for your continued existence, why not challenge theirs? Stop feeding them with your time and attention, stop justifying them by accepting their demands as valid. Instead remove yourself from their entanglement, sit back patiently and see that they dissipate without your continued support.
Your worldview needn't be coloured by every random thought that crosses your mind. Like storm clouds overhead, arguing with them won't change anything but if you simply wait them out eventually they move on.
I know the frustration of thoughts challenging your "right" to exist. But I've found considerable peace can be had once you learn your thoughts do not define you. The human mind is a crazy complex thing and random thoughts often burble up apropos of nothing. It is up to you to decide which thoughts you cling to and which you let evaporate.