[Salman]

For all that, the Sunni (for example the Saudis but also the Taliban) think the Shiite
Ayatollahs are dreadful hell bound heretics. Not because of any disagreement with the
above quotes, but a disagreement over which of Muhammad's relatives was the rightful
successor.

[Cronos]

Salman, the reasons Sunni consider Shia as heretics is varied:

1. Sunnis reject the Shia belief in the infallibility and divine appointment of the Twelve Imams, viewing it as an innovation (bid’ah)

2. Twelver Shia often criticize or curse certain companions of the Prophet, such as Abu Bakr, Umar, and Uthman, whom Sunnis revere as the Rightly Guided Caliphs; this is seen as disrespectful and divisive to the unity of the early Muslim community.

3. Some Sunni scholars accuse Shia of attributing human-like qualities to God (tashbih) through certain interpretations of Imams’ roles or divine attributes, which Sunnis see as deviating from strict monotheism (tawhid).