Answer: In order to answer your question the following points should be considered:
Imam Khomeini was totally rejecting any act of violence and terrorism, which are basically against the Islamic laws and jurisprudence. In this case also Imam just undertook to issue a decree in accordance with the laws of Islam on Rushdie's act, while others may directly go for execution and then take the responsibility.
Both Shi‘a and Sunni Muslim jurisprudents have concession and decrees on the issue of heresy. From this perspective, an apostate is the one who renounces a religious or political belief or principle, not a person who is doubtful about principles of religion.
Imam’s decree on Rushdie was supported from many of the Islamic scholars either in UK or in the entire Muslim world. It was regarded as a sign of protest and opposition to those who had fostered in mind to degrade the position of the Holy Prophet of Islam (pbuh) and show disrespect for the Muslim world sanctities.
Many committed Muslims after hearing the message of Imam tried to perform it through different ways. But severe security arrangements on behalf of the UK government and its American allied prevented them from Rushdie’s execution.
Therefore, Imam by his decree revealed the ill-nature of UK government to support Rushdie’s disgraceful insults written in his blasphemous book of the Satanic Verses.