Can a Muslim who damaged and harmed other Muslims and non-Muslims ask Allah for forgiveness?
God is all forgiving. It is His promise that when a person sincerely repents for his sins, He will not only forgive his sins - even if they are as grave as ascribing partners to Him, committing fornication and/or murder - but will also substitute these sins with good deeds in his record:
Except those who repent and believe and do righteous deeds; for those, Allah will change their sins into good deeds and Allah is Oft-Forgiving, Most Merciful. (25:70)
But there are two conditions of Tawbah which should be fulfilled: making amends for sins and a strong commitment to refrain from them. Tawbah is not merely the pronouncement of some words. It is a commitment that must be honoured. Therefore, a person should not only make a commitment not to tread the wrong path but also compensate for his cruelties and aggressions that he has resorted to. Hence, before seeking refuge in Allah, he should ask for forgiveness from his victims and also compensate them for the loss he caused to them.