It is important to note that Old Testament prophets were sometimes ordered by our great God to not show any mercy when carrying out God's orders. However, this was before the Messiah came and established his peaceful teaching and new covenant. No more are the followers of God to not show mercy. As Jesus Christ, the prince of peace of Isaiah 9:6, brought the new teaching of love and harmony for mankind, that is to be followed from now on.
"This I command you, that you love one another." John 15:17
"If, however, you are fulfilling the royal law according to the Scripture, "YOU SHALL LOVE YOUR NEIGHBOR AS YOURSELF," you are doing well." James 2:8
"In everything, therefore, treat people the same way you want them to treat you, for this is the Law and the Prophets." Matthew 7:12
"But flee from these things, you man of God, and pursue righteousness, godliness, faith, love, perseverance and gentleness." 1 Timothy 6:11
"Be at peace with each other." Mark 9:50
"But sanctify Christ as Lord in your hearts, always being ready to make a defense to everyone who asks you to give an account for the hope that is in you, yet with gentleness and reverence;" 1 Peter 3:15
"Finally, all of you, live in harmony with one another; be sympathetic, love as brothers, be compassionate and humble." 1 Peter 3:8
"Dear friends, let us love one another." 1 John 4:7
Under the new covenant, followers of God have been given a new universal moral law.
The actions of the great Old Testament prophets that Muslims often condemn do not make them guilty of breaking Jesus teachings, as many of Jesus commands are first century commands specifically meant for the new covenant peoples. Although the prophets of old were under a moral system, they were not yet subject to many of Christ's specific new covenant commands. Thus when they carried out God's will not showing mercy in some situations, they are not to be questioned, as back then they were doing the will of Almighty God.
So has Islam forgot about what Jesus taught since Muhammad came? Where's love, where's mercy?
I'll stick with Jesus who was here 600 years before Muhammad! Muhammad is no replacement for Jesus!