The Chafetz Chaim, in his sefer Ahavat Yisrael (chapter 1) explains why Sinat Hinam/Baseless Hatred is such a terrible Mitzvah.
He says that whenever a person commits a sin, he brings upon himself a spirit of contamination.
There are 248 limbs, and 365 sinews in the body, that each correspond to the positive and negative Mitzvot. Our bodies are a microcosm of the Mitzvot, and in order to be spiritually healthy, one has to keep the Mitzvot.
When a person does a mitzvah, it brings kedusha/holiness to a certain body part- for example when you do the Mitzvah of tefilin, it brings kedusha to your bicep, or if you have a mezuzah, it brings kedusha to your fingertips.
We don’t know exactly what matches what, but when you do mitzvot you become infused with kedusha.
This can be compared to how aerobic exercise brings health to the body, and how push ups help your muscles, etc.
But the opposite is true as well. When a person commits a sin, such as eating unhealthy food, consuming a lot of sugar, this brings unhealthiness, and in the spiritual realm, it brings contamination.
The Chafetz Chaim cites a Gemarah that if you listen to nonsense with your ears, your ears are spiritually impacted, and so on, with each of the limbs. With each sin comes tumah/ impurity, which sometimes brings a punishment. That is why when a person suffers a punishment, he’s told to check his deeds. Because based on a person’s deeds, certain organs can become corrupted or unhealthy.
The Chafetz Chaim continues that there are different types of limbs. A person can stub their toe or dislocate their pinky. This might be uncomfortable. A person might suffer from plantar fasciitis, which is when a muscle that runs under the foot gets inflamed. It’s extremely uncomfortable but not at all life-threatening.
But what happens when a person has a disease in a vital organ, such as the heart or brain? That could be deadly.
The Chafetz Chaim says that Sinat Hinam affects the heart. It’s a heart disease, and a heart disease is very dangerous.
For example, there is a certain bacteria, Endocarditis, which causes an infection of the heart. It often starts with a simple dental procedure such as a cleaning, but bacteria gets into the bloodstream and if the person has an artificial heart valve, it can be fatal in a matter of days.
The Chafetz Chaim says that Sinat Hinam is like a heart bacteria, and therefore it’s extremely dangerous.
We have to be very careful, and that is why we are working on our spiritual heart health.
Have a wonderful day.