i. Rabbeinu Bachya (1255-1340) explains that the mashchis operates under HaShem's authority. While HaShem directly intervened for Makkas Bechoros, He still used certain forces to carry it out and the mashchis was one of those forces. While the mashchis had to wait for HaShem's direct order for each Egyptian bechor, he had no specific restrictions against killing the Jews. Therefore, specific measures had to be taken to ensure that the mashchis didn't kill any Jews. (Shmos 12:23)
ii. R. Chaim ibn Atar (1696-1743) explains along similar lines that HaShem did employ specific forces to carry out Makkas Bechoros. However, because it is difficult to determine who really is a bechor and who is not, HaShem had to guide these forces and instruct on an individual basis who the mashchis can destroy. (Shmos 11:5)
iii. R. Akiva Eger is quoted as saying the following: the firstborn of the mother is well-known. However, the other types of firstborn are not well-known. HaShem allowed the mashchis to kill the firstborn of the mother, but not the other types of firstborn, which require much more scrutiny. Therefore, only Jewish firstborn from the mother are sanctified. They were the ones who were saved from the mashchis who doesn't normally distinguish between Jews and Egyptians. (Chut haMeshulash page 182)
iv. R. Yosef D. Soloveitchik (Beis HaLevi, 1820-1892) suggests that after Makkas Bechoros, there was major plague caused by the mass exposure to dead bodies. During this plague, many Egyptian non-bechoros died. This is why the verse states that every household had a death in it. This plague was carried out by the mashchis and the Jews were spared from it. Makkas Bechoros was limited to bechoros. (A chapter from Festival of Freedom)
v. R. Aharon Kotler (1891-1962) suggests that perhaps the role of the mashchis can be explained based on the comment found in the Haggadah that each makkah really contained five sub-makkos. R. Kotler suggests that the first four sub-makkos were carried out by the mashchis. The fifth, which was death itself, was carried out by HaShem. The Jews were spared from the first four sub-makkos carried out by the mashchis. (Hagadah Shel Pesach page 60)
vi. R. Yitzchak Zev Soloveitchik (1886-1959) suggests that when we say dayeinu for Makkas Bechoros, it is qualitatively different than the praise we say for judging the Egyptians. The ten makkos as a whole was a form of justice. It was a fulfillment of the Bris Bein HaBesarim when HaShem said וגם את הגוי אשר יעבדון דן אנכי. However, Makkas Bechoros served as the means that caused yetzias Mitzrayim. Without Makkas Bechoros, we would have never been freed. This is why we specifically mention Makkas Bechoros in Hallel HaGadol. Hallel HaGadol only discusses the actual exit from Egypt, not the miracles that led up to the Exodus. Therefore, only Makkas Bechoros, which was part of yetzias Mitzrayim, is mentioned. (Hagadah Shel Pesach page 178)
