What is "l'Mashchis"?
Radak: It is total destruction.
How will they begin from the Mikdash?
Rashi: They will strike those standing in front of the Mikdash.
Rashi citing Shabbos 55a: We read "umi'Mikdashi" like 'mi'Mekudoshai (my holy ones)', Tzadikim with a Tav written on their foreheads 1 .
Malbim (the Nimshal): Due to the sins of the generation, Sarei ha'Am were authorized to kill in the city. However, Hashem commanded Yirmeyah to go in the city to get them to repent. He will write a Tav on those who heed him - "veha'Yotzei El ha'Kasdim v'Chayah v'Haysah Lo Nafsho li'Shlal va'Chai" (Yirmeyah 38:2). They will be saved - "Al Tigashu." Those who corrupt will be killed in the city - "Hineni Mesev Es Klei ha'Milchamah Asher b'Yedchem...; v'Nilchamti Ani Eschem" (Yirmeyah 21:4-5). Hashem will make them fight each other. The rebellion will begin in the Mikdash when the nation gathers there; they began to strike the Zekenim in front of the Bayis.
Shabbos 55a: Even though it said "v'Al Kol Ish Asher Alav ha'Tav Al Tigashu", Hashem retracted due to Midas ha'Din; the Tzadikim should have protested against the sinners. (Alshich, brought in ha'Rif - the criticism and decree were only against Tzadikim with authority who could have protested.)
Why were the Zekenim "Lifnei ha'Bayis"?
Malbim: They were appointed there, or they served Avodas ha'Mikdash there.
