Wa alaikum assalam,
This phrase is not atypical and it is found elsewhere in the Quran. If we note the fundamental right of equal lawful retribution in the case of murder expressed in the Torah we note the Arabic expression, 'al nafsa bil nafsi' i.e. the life for the life. This expression is almost identical to those found in verse 2:178 and underscores the principle of interpretation which is that only the life responsible that has committed the murder must be held responsible. (The life for the life) Of course the Quran also allows for just compensation as mitigation so that life does not need to be taken as retribution (2:179).
Taking the life (or lives) of an innocent party in exchange of another as you rightly infer, is not a concept that can be supported by the Quran. Hence the Arabic phrases found in verse 2:178 are best understood from its own context, similar expressions found elsewhere in the Quran and primarily through the lens of the relevant underlying theological teachings of the Quran.
To strengthen this point, if in the relatively minor case of stealing, only the person responsible is to be held accountable (12:78-79), it is inconceivable that in the case of murder (where life is sacred) that anyone else but the guilty party is responsible.
“He said, God forbid! That we take anyone else except the one with whom we found our possession. Indeed, we then would surely be the wrongdoers” (12:79)
Equal retribution is not only the sine qua non of the Quran's teachings but of all God's scriptures. After all, the same fundamental 'deen' was inspired in all the Prophets (42:13) and the law of equal retribution can be found in earlier teachings.
"And We prescribed for them therein: The life for the life, and the eye for the eye, and the nose for the nose, and the ear for the ear, and the tooth for the tooth, and for wounds is retribution. But whoever forgoes it (in the way of charity) it shall be expiation for him. And whoever does not judge by what God has revealed, then those are the wrong-doers." (5:45)
Justice is only upheld if retaliation is meted out to the extent of the violation. This principle of justice is also seen in verse 16:26.
"And if you punish/ retaliate, then punish / retaliate with the like (bimith'li) of what you were afflicted. But if you endure patiently, verily it is better for the patient." (16:126)
Therefore in my humble opinion, a better rendition of verse 2:178 would be for example "If a free person has committed murder, that free person will face retribution" or "if a freeman is guilty then the freeman, if a slave is guilty then the slave, if a female is guilty, then the female" and so on.
Any secondary source which attempts to skew interpretation otherwise cannot be accepted in light of the Quran, which remains unequivocally the final criterion to judge.
I hope that helps, God willing.
Joseph