Actually I haven't played any Naruto game,when Amaterasu hits Pain,Pain will be done by it and even if what you said is true,Itachi will burn the others Pain,Preta path "The fat Pain" can't catch up with Amaterasu speed which is eye sight speed
and what then???The God (Pain) keep running away???
As I said before he didn't kill anyone by it because he is peaceful and just acting like enemy against konaha's Ninja,he murdered his own family for konoha sake for peace,when he cost it on kakashi,Kakashi was alomst dead by it but Itachi stopped it because he wasn't aiming to kill him even kakashi was wondering about Itachi didn't kill him.
as I said before it's godlike jutsu if there wasn't side effect,and yes it's required more chakra but Itach's chakra is high level not like Kakashi which is low-average chakra level,Itachi was dying and the disease was eating him from inside and he used many jutsus when he was fighting saskue,he used all his Mangekyo Sharingan jutsus "Amaterasu,Susanoo and Tsukuyomi" he used also others jutsus like shadow clone and katon,all that he was ill so that it's proved that Itachi is capably of using Mangekyo Sharingan for long time if he was healthy due to his high amont of chakra.
For god sake just read my comments carefully,I have mentioned Demon realm before you.
Yes,he has long range attack as I said
Man that's Shinra Tensei jutsu,it's not long range attack it's close range, look carefully kakashi was attacking him closely when he used.
Itachi isn't stupid,he is smart he know how he used it wisely without losing it to a summoned creature and even Pain can't afford to lose his summons to an explosion clone.