Parsha: Lech Lecha, "Leaving Israel"

In  Mishna Torah: Hilchot Melachim: 5:9, the Rambam writes that:


"Even though it was a famine and one is permitted to leave Israel [nevertheless] "einah middat hassidut" ... [It is not the mark of a pious character]
And because of that Machalon and Kilyon - 2 "gedolei hador" [great ones of their generation] ..left and were liable to destruction by "the Maqom".

Questions:
1. If it's Halachically permitted to leave the land of Israel, then who cares about "Middat Hassidut"?

2 If all Jews are on the same level of obligation and liability, then how does being a "Gadol Hador" change someone's level?

3. If even the Rambam concedes that Machalon and Kilyon's leaving Eretz Yisrael was Halachically permitted, then why did he say they were punished? 

4. In Lech L'cha, Avraham Avinu left the Holy Land under similar circumstances - and he seemed to be rewarded and not punished! How does this Rambam address that case?

For further sources

• Torah Temimah on Lech L'cha 12:10 quoting Bava Qama 60b
• Note 13 quoting Bava Batra 91a

Shalom, RRW