There is a machlokes between the Ketzos (2:1) and the Tumim whether the rule of Goel HaDam applies today, namely, without a determination of law by a court that is empowered to judge capital cases.
First the Xos brings the Tumim's opinion that the legal entitlement of the Goel HaDam occurs only upon a decision of the Beis Din to so empower him. The Tumim bases his opinion on the words of the Rambam in 1 Rotzei'ach 5:
The Ktzos argues and says no such empowerment is necessary; being a Goel HaDam is an extra-judicial status. The Rambam the Tumim brought is not talking about the Go'el Hadam, it is talking about court administered punishment. (He also notes that the Rambam and Rashi argue about the extent of the law of Goel HaDam: Rashi applies it even to inadvertant but negligent manslaughter, while the Rambam requires some degree of criminal negligence.) The Ktzos does, however, agree that it is possible that a court's involvement is necessary simply to establish the facts of the case- על כל פנים בעינן קבלת עדים להודיע אמיתת הדבר שהרגו הרוצח.
Reb Shmuel Rozovsky, in his shiurim on Makkos, disagrees with the Xos' interpretation of the Tumim. It is possible that the Tumim is saying that Goel HaDam is a MITZVA only when the court determines that the killing was intentional but a procedural barrier to capital punishment prevents justice from being done. The Tumim might agree with the Ketzos that even in other cases, and even without a finding by the court, the Goel HaDam would not be guilty of a capital crime if he killed the Rotzei'ach. Pattur- yes, maybe even a Reshus, but Mitzva- no.
This reminds us of the Mishna L'Melech (1 Rotzei'ach 15):
Clearly, then, the rule of Kana'im is totally extra-judicial. What about Goel Hadam? Does the Rotzei'ach have the right of self defense? Will the court interfere if he hires mercenaries, or enlists his friends, and creates a militia to defend him from the Goel Hadam? And does the rule of Goel Hadam pertain in out times, when our Batei Din are no longer empowered to judge capital cases?
So I want to point out something interesting about the characterization of the act of the Goel as respectively a Reshus or a Mitzva.
- In Makkos 10b, Rav Huna says that if the Goel finds the Rotzeiach outside the Ir Miklat and kills him, he is Pattur. אמר רב הונא רוצח שגלה לעיר מקלט ומצאו גואל הדם והרגו פטור. The implication is that he is merely pattur, but he has no mitzva.
- The Mishna on 11b brings a Machlokes Reb Akiva and Reb Yosi HaGlili as to whether the Goel that finds the Rotzeiach outside the Ir Miklat has a mitzva or a reshus to kill him. רוצח שיצא חוץ לתחום ומצאו גואל הדם רבי יוסי הגלילי אומר מצוה ביד גואל הדם ורשות ביד כל אדם רבי עקיבא אומר רשות ביד גואל הדם וכל אדם חייבין עליו .
- It appears that Rav Huna is consistent only with Reb Akiva's opinion, which is impossible, because the Gemara would have noted that.
The Divrei Yechezkel (which I am reprinting pursuant to my father zatzal's will if the twit who thinks he owns the rights doesn't create too many problems) in 23:8 and slightly differently in the Chazon Ish in Likutim 23 to Makkos 10b both say that Rav Huna's expression Pattur only applies where the Rotzei'ach fled to the Ir Miklat before any decision by the Beis Din. The Machlokes Reb Akiva and Reb Yosi HaGlili is where Beis Din already paskened Galus on the Rotzei'ach, and then he left the Ir Miklat.
Here is the Divrei Yechezkel, albeit messed up by the OCR. Good luck figuring it out.
Here is the Chazon Ish, which I mostly fixed up. The Chazon Ish distinguishes between a Hora'a of Beis Din and a Psak of Beis Din.