Who is "Malkam"?
Rashi: The idolatry of Amon is called Malkam. It is the same as "Molech Shikutz Bnei Amon" (Melachim I, 11:7).
Radak: It is the king of Amon.
How could David's head bear the crown, which weighed a Kikar (about 25 kilos)?
Radak #1: It hung suspended above his head when he sat on his throne.
Radak citing Avodah Zarah 44a #1: (It did not weigh a Kikar.) It contained a gem worth a Kikar of gold.
Radak citing Avodah Zarah 44a #2: It was a magnet (this supported most of its weight).
Radak #2: It was on David's head occasionally. He could bear the weight for a short time.
Avodah Zarah 44a #3: The verse means, it was proper to be on David's head.
Kli Yakar: It was made from a Kikar of gold after the Kikar was refined, which diminishes it. 1
Kli Yakar: The crown weighed a Kikar. Only the gem from it was on David's head.
Yerushalmi (Shekalim 6:3): "Mechalos Zahav" (Divrei ha'Yamim II, 4:21) - Shlomo put 1000 Kikarei Zahav into the furnace, and one Kikar of super-refined gold came out.
Rashi (citing Avodah Zarah 44a) writes that Itai ha'Giti, a Nochri, nullified the idolatry. What is the source that he was a Nochri?
Radak: His name is not a source. Oved Edom ha'Giti was a Levi. He is called Giti because he lived in Gas. Many Jews lived there. When David fled Avshalom, 600 Gitim came to help him (15:18). Can we say that they were all Pelishtim 1 ?! It mentions also their children (15:22)! Surely they were Jews. They lived in Gas before David took it from Pelishtim, or afterwards. David said "you are a Nochri" (15:19), i.e. from another land, for he lived in Gas. Why would he tell him "return with the king (Avshalom, who seized power)" because you are a Nochri 2 ?! We find that Nochri can refer to Jews from elsewhere, e.g. Rus said "v'Anochi Nochriyah" even though she had already converted - "Amech Ami vEilokayich Elokai" (Rus 2:10, 1:16)! Why would David need specifically Itai to be Mevatel the idolatry? He had many Jews ruling over Nochrim. One of them could force a Nochri to be Mevatel it! 3 And how could Itai be with David so many years and not convert 4 ? This is astounding.
He cut off 200 Orlos of Pelishtim, they knew that women used to sing that he kills myriads (1:29:5), and he struck Pelishtim after he became king (5:20)! Otzar ha'Midrash Eisenstein, Yisbarach p.251 says that the 600 were Nochrim, and they clung to Yisrael (converted. Only after David rose to power, we did not accept converts - refer to 12:30:4:1**** - PF.)
Rashi (15:19): We are fleeing without provisions. You are a Nochri, so people will not have mercy on you!
This is like Chasam Sofer (Chulin 8a), who infers from Avodah Zarah 53b that one may take idolatry with intent to get a Nochri to be Mevatel it afterwards. (PF)