Who were the five kings (with the suggestive names)?
Rashi: Bera was wicked to HaSh-m and wicked to his fellow-men 1 ; Birsha excelled in his wickedness; Shin'av hated his Father in Heaven and Shem'ever made himself wings to fly (Sam Eiver), in order to rebel against Ha'Kadosh-Baruch-Hu.
Targum Yonasan: Bera was wicked; Birsha was evil; Shin'av was hated by his Father in Heaven and Shem'ever corrupted his 'Ever' (male organ) through his promiscuous activities.
I.e. Beis Ra, "twice evil."
Why does Rashi interpret the names of these kings as pejorative nicknames highlighting their wickedness? He did not do so for the four kings in the previous verse!
Gur Aryeh: The Torah does not even tell us the name of the king of Bela (Tzo'ar). The other kings of this verse must be named only to tell us their wickedness, whereas the city of Bela was not as evil (by comparison, 1 therefore its king is unnamed).
See Rashi to 19:20.
Why does the Pasuk not give the name of the king of Bela?
Seforno: Because it was only a small town (as the word "Tzo'ar" implies). Consequently, he was not well-known like the other kings were.
If the name of the city was Tzo'ar, why is it called Bela?
Targum Yonasan: Because the land Bala (swallowed up) its inhabitants. 1
See Sefer 'Na'ar Yonasan'.
If the name of the city was Bela, why is it called Tzo'ar?