Rashi #1: Even though the Makos were sent with Af (anger), they did only their Shelichus. What they were commanded to kill, they killed, but not everything; they went in their paths.

Rashi #2: When He killed the Bechorim of Egypt, He commanded the destroyer with anger to go to the houses of Egyptians, but not of Yisrael.

Radak: Hashem sent His anger straight at them, without delay. This is like "Pales Magal Raglecha" (Mishlei 4:26).

Malbim: Also Makas Bechoros was not needed. He could have killed them all at the time of Dever - "Ki Atah Shalachti Es Yadi va'Ach Osecha v'Es Amecha ba'Daver va'Tichached Min ha'Aretz" (Shemos 9:15)! Rather, Hashem's anger, which is the Dever that came out, its matter was to destroy all of them. Hashem weighed a special path for it. Nesiv is a narrow path for an individual. The Dever will not go in an encompassing path to destroy people; this He left for Makas Bechoros. Dever killed only Chayasam (their animals), but not people, in order to multiply His wonders (additionally send Makas Bechoros).

Radak: There is no need to mention that He did not withhold from death those whose time [to die] arrived. Rather, He closed (prevented any escape from) death without their time. This refers to Makas Bechoros in the coming verse.

Malbim: His anger would not have withheld death from [the Egyptians]. Therefore, He sent it on a narrow path to limit it to their animals, to enable also Makas Bechoros.

Rashi: It is their bodies.

Radak #1: It is Nafsham.

Radak #2, Malbim: It is their flocks. (He closed death on them.)