There are certain cases in which it is appropriate to kill someone, and certain cased in which killing is inappropriate. Typically appropriate killing would be legal executions. For discussion of when extrajudicial killing is justified see here. The quote doesn't say anything about which cases are which. Rather, it says that killing certain people is considered a great sacrifice. Presumably this refers to those people whom it is obligatory to kill. Indeed, the case at hand is one of the few cases in which killing may be justified (see the link).
Also noteworthy in context is that as with most rabbinic quotes, they say more about the individuals saying them, then Judaism as a whole. This quote, for example, is rather obscure. Searching through a database of thousands books (Bar Ilan Responsa project version 23), yields just 11 results. It is therefore hard to clarify "the Jewish approach to it", given that it is an obscure statement without much clarification. We can, however, clarify which cases warrant extrajudicial killings, and which do not. The above link shows the limits of extrajudicial killing.
Indeed, statements about the value of human life, even the value of the lives of the wicked, are much easier to find in Jewish literature. For example, Man is created in the image of God (Genesis 1:27). Additionally, even the bodies of executed criminals must be treated with certain dignity (Deuteronomy 21:22-3). The Talmud recounts that the angels wanted to praise God for destroying the Egyptians in the Sea of Reeds, but God angrily retorted: "My creations are drowning in the sea, and you want to sing praise?!" (Megillah 10b, Sanhedrin 39b). Additionally, the Mishna (Makkot 1:10) quotes R. Elazar ben Azarya as stating that a court that executes someone every seventy years is considered violent. R. Tarfon and R. Akiva say that if they were on the court no one would ever be executed. And speaking of the spilled blood of the wicked, the Mishna states that God mourns the spilled blood of the wicked (Sanhedrin 6:5, quoted in Hagiga 15b and Sanhedrin 46a).
The quote in question need not necessarily contradict these classical Jewish sources (although it does unquestionably have a different tone). Indeed, Rabbenu Bahya himself references the teaching of Sanhedrin (6:5) in his commentary to Deut. (21:23). There are unquestionably cases in which someone must be killed. That in those cases, carrying out this deed is meritorious, doesn't seem so problematic.