There is a hadith that says that if you intend bad but do not do it you get one reward. Why then, in the hadith where two Muslims fight, do they both go to hell because they intended to kill?

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger. 

First of all, there is no contradiction between these ahaadeeth; if a person intends to commit a sin but did not commit it out of fear of Allaah, then he will be rewarded, and him abandoning the sin is counted as a good deed.

However, if a person abandons the sin because he was unable to perform it while he was determined to do it if he had the ability to do so, then he is held accountable and is sinful. So there is a difference between the two kinds of abandoning the sin: one kind is rewarded, and the other kind is punishable.

An-Nawawi said:

“The hadeeth reads, 'Allaah says, 'Whenever My slave intends to do a bad deed, do not record it against him until he (actually) commits it. If he commits the sin, write it down as one bad deed in his record book. And when he intends to do a good deed but does not actually do it, write it down as one good deed for him. And if he actually does it, then write it down for him as ten good deeds.'' Another hadeeth reads, '…(write it down for him) from one good deed to seven hundred times.' Another hadeeth reads, '…he only abandoned the sin for My sake.'

Imaam Al-Maaziri said: 'The view of Al-Qaadhi Abu Bakr ibn At-Tayyib is that if a person has intended in his heart to commit a sin and he was content with the idea (of doing it), then he is sinful by thinking in this manner and in his being determined to commit the sin. These ahaadeeth and similar ones are understood to apply to a person who had a thought to commit the sin without accepting the idea in his heart or being determined to do it. There is a difference between wanting to do something and being determined to do it. This is the view of Qaadhi Abu Bakr, but many jurists disagreed with him, as well as many scholars of Hadeeth; as they interpreted the hadeeth according to its apparent meaning.'

Qaadhi 'Iyaadh said: 'Most scholars from the Salaf, and the jurists and the scholars of Hadeeth are of the same view as that of Qaadhi Abu Bakr because of the ahaadeeth that indicate that a person is held accountable for the actions of the heart, but they said that this determination is written for him as a bad deed, but it is not the bad deed that he intended to do and he did not do it because he was prevented from doing it by some reason other than the fear of Allaah and repentance. Rather, it is the determination itself that is a sin, and it is recorded as a sin; and if he actually did it, then it is written as a second sin, and if he refrains from doing it out of fear of Allaah, then it is written as a good deed as in the hadeeth, 'He refrained from doing it for My sake;' so he refrained from doing it out of fear of Allaah and striving against his own self, which orders him to do evil, and he disobeyed his desires.'

The thought that is not recorded is the thought that a person had but without having any determination or intention to do it.

Some people stated a difference of opinion in regard to when a person refrains from doing a sin but not out of fear of Allaah, but, rather, because of fearing people, will it be written for him as a good deed? They said, 'No, because the reason why he did not do it is because he was shy [from the people];' but this view is weak, and it is not a considerable opinion. This is the conclusion of the statement of Al-Qaadhi, and it is a reasonable opinion.

There are many Islamic texts that indicate that a person is held accountable for being determined in his heart to do something, among which is the saying of Allaah (which means): {Indeed, those who like that immorality should be spread [or publicized] among those who have believed will have a painful punishment in this world and the Hereafter. And Allaah knows and you do not know.} [Quran 24:19]

Allaah also says (what means): {O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin.} [Quran 49:12]

There are many verses in this context, and there is a consensus of the scholars that envy, contempt for the Muslims, and wishing evil for them is forbidden, as well as other acts and determinations of the heart.”

Allaah knows best.