Chazal (Bava Kama 60:) say, "If there is a plague (dever) in a city, one shouldn’t be alone in a beis kneses, because the malach hamaves stores his tools there." Derech Moshe (on Sefer HaGan) asks why the malach hamaves store his tools precisely in the beis medresh. It's because the malach hamaves gets his strength to harm people from those who speak in the beis medresh."
Shulchan Aruch (124:7) states, "Don't speak idle talk during chazaras hashatz. If one speaks, he is sinning and מנשוא עוונו גדול, the sin is too great to be forgiven,' בו וגוערין, and you should shout at him.'"
These harsh terms, מנשוא עוונו גדול 'the sin is too great to be forgiven' were originally said by Kayin after he killed Hevel. This expression describes the severity of speaking during davening, and Shulchan Aruch doesn’t use this expression for any other aveirah. Is there an association between Kayin's murder and speaking during tefillah that they are both called מנשוא עוונו גדול? The answer is, speaking during davening can also cause deaths, r'l, since the malach hamaves receives his powers to cause plagues and epidemics because of the people who talk during tefillah.
Shulchan Aruch states בו וגוערין, "People should shout at him." People asked Reb Shlomo Zalman Auerbach zt'l whether one should do so literally. He replied, "If you saw someone killing his fellow man, wouldn’t you shout at him?"
Some say that refraining from speaking during tefillah is a לעמוד יכולין הציבור רוב שאין גזירה בו, a law that most people are unable to maintain. We ask them, are the tzaros that come from speaking during davening something the public can sustain? These tzaros are also בו לעמוד יכולין הציבור אין, something people simply cannot endure. If we stop talking during davening, we will be freed from all these tzaros, and our lives will be much better.
The Derech Moshe writes
שיבה, old age, is roshei teivos התפילה בשעת יפה שתיקה, "Silence is good during tefillah." This means those who are silent during the tefillah will merit to live long.
The Debreciner Rav zt'l (Be'er Moshe 7:108) writes, "I will tell an awesome story. I saw it myself. My feet still tremble when I think about it. One Shabbos, my father zt'l hy'd delivered a drashah on the importance of answering amen and amen yehei shmei rabba. He repeated the Remak that שיבה is roshei teivos for התפילה בשעת יפה שתיקה, that in the merit of being silent by the tefillah, one will live long. My father added, 'The opposite is self-understood (that if people speak by the davening, they won't live long. He didn’t explicitly say those words, but the implication was understood.)
"The next day, there was a commotion in the beis medresh because one of the people who heard my father's drashah said that in his dream, he saw himself standing before the Heavenly tribunal, and the court ruled that he was guilty (חייב). He was still asleep but understood that he wouldn’t wake up in the morning. He pleaded before the court that he hadn't yet reached שיבה. Why should he be punished? Heaven replied that it's because he spoke devarim beteilim during the tefillah and didn’t answer amen and amen yehei shmei rabba. He replied, 'I didn’t know the matter was so grave.'
"One of the judges became angry with him and said, 'Why do you say that? On Shabbos, your Rav taught you about the importance of these matters. He said in the name of the Remak that those who are silent in beis medresh and they answer amen and amen yehei shmei rabba will merit שיבה, and he said that the opposite is self-understood... You were present at this drashah... How can you say you didn’t know the severity of this sin? The man declared to the beis din that he would tell the entire community what happened to him in the Heavenly Court, and from now on, he will warn them to be silent in beis medresh and to answer Kaddish, amen, and amen yehei shmei rabba. Upon that, the beis din permitted him to live. Everyone who heard the story was afraid."
