Does Jesus forgive a Christian believer, even if he or she has killed many innocent people?

Answer:

Yes, Jesus Christ is willing to forgive the sinner, whatever sin he or she may have committed. However: true repentance is the condition. This means conversion of heart, a turning away from evil and a radical determination to make a new start. In speaking of the Sacrament of Penance we stress the firm intention of making amends for the wrong he or she has done. We use the word ‘confession’ to describe the individual’s own personal confession of his or her sins. We also use the term ‘Sacrament of Reconciliation’, to show that God reconciles us to himself. 

 For further information see: Topic 3: Cross , Sin, Redemption, II: Christian Perspective 1 (ii) and  Questions and Answers Nos. 193; 192; 241; 257.