Real Muslim is the person who believes in Islam and behaves in peaceful way with people (المسلم من سلم المسلمون من لسانه ويده))
Every Muslim is required to practice Islamic teachings in his life especially when dealing with people. Those Muslims who do not practice fairness and justice with others are blames in Quran ( O, Those who believed, why do you say what you do not practice? (Sura Al-Saff, Verse 2).
As being nice to people is one of the best deeds in Islam, being bad and harming people is very bad and it can lead tp punishment in this life and hereafter. When harming an animal is forbidden in Islam (e.g. The Hadeeth about a Muslim woman who will be in hellfire because she tortured a cat), so what about harming a human being?
No one from us can judge who is good Muslim and who is not as Allah (SWT) knows what all creatures did and ae doing and will be doing in the future and He judges who is good and who is not. But Allah (SWT) said clearly (The most honorable of you with Allah is the most practicing Taqwa (piousness). Sura Al-Hujuraat, verse 13.
Those who pray and fast and worship Allah (SWT) but misbehave and harm others will loose their worship reward to their victims, and might be forced to take the sins of their victims.
The Hadeeth says ( Best of people are those who benefit people) and another Hadeeth says (Worst of people are those who harm people).
This shows that the spirit of Islam is to be good to others.