TT #219 · Parashas Shoftim · Fifth Aliyah · Passuk 19:13
Your eye shall not pity him; you shall purge the innocent blood from Yisrael, and it shall be good for you.
Torah Temimah points to Sifrei Devarim 186 / Makkos.
TT lemma: לֹא תָחוֹס עֵינְךָ.
Sefaria — Sifrei Devarim 186 / Makkos
English: ‘Your eye shall not pity him’ – that you not find grounds to acquit him.
Derash against misplaced pity (Sifrei) — the verse forbids shielding the deliberate killer.
One must not pity the intentional murderer; the innocent blood must be purged from Yisrael so that it may be well with the people.