If he was posessed by a Demon, He could not have been saved since the Holy Spirit takes residence in the body of a believer and will not tolerate a demonic presence.
Therefore I would say with a high degree of certainty that Mr. Hui went to hell. the only possibility that he could have gone to heaven, was if he repented and accepted Christ as Saviour.
But I find this unlikely from the testimony of the witenesees that said he turned the gun on himself immediately after killing.
There is also some rigorous doctrinal arguments that once a person is saved with a regenerated human spirit, they are always saved and belong to Him. It still remains possible for the believer to backslide and fall into degeneracy, becoming susceptible to demonic influence (not possession), and in such a degenerate stage perform any number of criminal and insane acts, including murder and suicide. In such a backsliden state, the carnal believer simply fails to advance in sanctification, and scars their soul becoming more susceptible to sin and its consequences of evil and human good separate from divine good.
It isn’t fruitful to ask if a person is saved for heaven or lacking salvation, because that lowers Christ to man rather than man to Him via faith through Him. Instead we simply remain in fellowship with Him through faith in Christ by simply returning to Him and confessing any known or unknown sins to God through faith in Christ and He is sure and just to forgive us those sins, returning us into fellowship with Him again. (see 1stJohn 1:9)
The ‘Ismail Ax’ penname does suggest a common tendency amongst Muslims to attempt to have relationship with God, independent of faith through Christ.
This results in tendencies to confuse morality with salvation or sanctification. The person who refuses faith in Christ has nothing of righteous merit with which God in His immutable nature of righteousness may have relationship.
Perfect righteousness may only have relationship with perfect righteousness. Any association with even the slightest of unrighteousness demands perfect justice.
This manifests a difference between faith through Christ and any other attempt to have a relationship with God.
We know faith through Christ is considered righteous by the Father because it is the same as the faith if Christ on the Cross (Rom 3:22-24).
Many Muslims might attempt to have a relationship with God by attempting to perform some work or exercise a faith in justice rather than simple faith in Christ. If their faith is the same as Christ on the Cross, then they have a surety of a saving faith. Unfortunately, as one becomes mired in sin, and attempts to find justice without faith in Christ, there will be a strong temptation to promote justice of unrighteousness whereever it is observed.
IMHO, I suspect in the case of this murderer, he may have indeed observed unrighteousness in others. He may have allowed his thinking to begin judging that unrighteousness he perceived without faith in God by God’s method of handling issues. This led to a number of arrogant thinking processes, which when left unchecked simply consumed his thinking to the point of criminal behavior and ultimately leaving him good for nothingness.