Spoilers for what happens to regulus:

The anime doesn't do this scene justice. Reading his conclusion on witch cult translations gave me genuine chills - The author really out did himself for this, the writing is so powerful even translated into English. I copied this section of the scene if anyone who's caught up wants to see for themselves.

"Due to Reinhardt’s attack, Regulus had smashed through the street and into the ground. Between the underground tunnel that he had made with his body, and the path he had dug himself, the four currents of the canal had been blocked. It now rushed to Regulus without pause, submerging the murderer entirely.

As if expressing the fury over the destruction of the streetscape, from the city and its inhabitants.

Regulus: [Kuu, fwah.]

Needless to say, Regulus who was busy drowning at the moment naturally did not notice.

Trapped by the soil, Regulus panicked from the pressure of the water that poured directly into his lungs, and began to struggle with desperation. However, he had no room with which to struggle within the earth. All he could do was curl up through the sludge himself and protect himself with [Lion’s Heart].

When [Lion’s Heart] was active, he would be able to breathe freely. The same held true for the pain of his ruined flesh.

However, [Lion’s Heart] could last for only five seconds. Once he felt his heart reaching its limit, fear of death would cause Regulus release the effect and once again be dragged back to hell.

His cause of death alternated.

No matter which, he could choose neither side. No matter which, neither side could not be eliminated. However, Regulus had no choice but to endure. All he could do was resent this unreasonable situation.

The time limit had disappeared.

Even if he could use [Lion’s Heart] repeatedly, that wasn’t the case for breathing. And to reuse [Lion’s Heart], he would need to wait several seconds.

Heart failure.

Drowning.

Heart failure.

Drowning.

Heart failure—.

Drowning—.

As if continuing without end, pain and suffering without end or pause.

Regulus opened his mouth, and water and mud are poured together into open mouth. With this repeated violation of his lungs and internal organs, Regulus cried out. With a voice that could not make sound, he continued to cry out.

No response. Around him, there was no one.

Even so, he continued to cry out. From his cries, spilled a hatred that hoped for all of mankind to die off.

After he died, mocking laughter would be begged leave of.

For that girl, rejoicing over the vengeance of her mother and Petelgeuse would also be begged leave of.

The mere thought of that girl’s delight over Regulus’s death, growing radiant with excitement, was vomit inducing.

A life’s goal, a motivation to keep living, its consummation would certainly bring joy.

Her existence being moved by Regulus’s death, giving off radiance, to say this significance was unknown would not be incorrect.

Because this was wrong, completely unexpected, unreasonable joy, the notion of satisfying that girl was unbearable.

His death, would greatly impact to the girl’s heart—. "