1)
Why does it say "va'Yaku" (plural) and "va'Yames" (singular)?
1.
Malbim: They [all] struck him, and Yishmael completed the death.
2)
Why did they kill Gedalyah?
1.
Rashi (1): The officers were jealous of him.
2.
Radak: Yishmael was jealous that he was appointed over the land 1 .
3.
Abarvenel: Melech Bnei Amon sent him, for he hated Melech Bavel, and did not want Eretz Yisrael to be under the rule of his appointee. Malbim (1,2) - also 2 , Yishmael thought that he himself should reign, for he was from Zera ha'Meluchah.