Please distinguish between teshuva and kapara. Kapara is what is required to restore the Torah-governed balance of things that was upset by a wrong act (kapara, from kaf, kipa – to cover over something). To get a kapara for killing someone may be extremely difficult and depending on the circumstances, the kapara may not occur until the killer himself dies.
Teshuva is a different story. The killer can better himself and remedy his own personal misconduct by regretting his wrong act, resolving to abstain from it in the future, and actually mending his ways when a similar situation presents itself in the future.