If an onen comes across a meis mitzvah (let's assume there is either another person to be involved with the onen's personal meis or there is more time to be involved with his meis later), is the onen allowed to be involved with the meis mitzvah? Is it optional for the onen? Obligatory? Perhaps ossur? What's the proof for this?
The first Rashi on Daf 20a in ברכות teaches us the underlying principal of Meis Mitzva:
אבל מטמא הוא למת מצוה. והוא כבוד הבריות, ודוחה דבר תורה
A Meis Mitzva takes precedence over other Torah-prescribed activities - including prohibitions and positive Mitzvoth - because of Kovod HaBrioth, dignity to humans.
The Gemara there - and in other places (like Nazir 47a) - analyses why certain people have to bury the Meis Mitzva, including:
- A Cohen Gadol who is never allowed to become Tamei. He has a positive commandment to remain in a Tahor state, and negative commandments not to become Tamei.
- A Nazir who has to restart his Nazir count - and bring an atonement for transgressing a negative commandment of not becoming Tamei- if he becomes Tamei.
- Somebody on his way to bring Korban Pessach - who could get the punishment of Karet if he doesn't fulfil the positive commandment to bring the Korban Pessach on time.
- Somebody on the way to give his son a Brit Mila; a positive commandment with Karet consequences of unfulfilled.
Similarly we see in Daf 3b of מגילה that Meis Mitzvah takes precedence over prohibitions:
גדול כבוד הבריות שדוחה את לא תעשה שבתורה
There the Gemara defines precedences:
- Meis Mitzvah takes precedence over Torah study.
- Meis Mitzvah takes precedence over עבודה - bringing sacrifices.
- Meis Mitzvah takes precedence over Megila reading, despite being Pisumei Nisa - advertising the miracle - which takes precedence over almost everything else.
That said, the Gemara on 7a in יבמות mentions that Shabbat observance has precedence over Meis Mitzvah.
Even if we assume that the Torah forbids an Onen from doing Mitzvoth, nevertheless Kovod HaBrioth would override that - as we see that it overrides positive and negative commandments.
He would therefore be obligated to bury the Meis Mitzva, possibly even at the expense of delaying his own relative's burial.