Not true, God used murderers mightily. Paul used to murder Christians.
Acts 7
57 Then they cried out with a loud voice, stopped their ears, and ran at him with one accord;
58 and they cast him out of the city and stoned him. And the witnesses laid down their clothes at the feet of a young man named Saul.
Acts 13:9 Then Saul, who also is called Paul, filled with the Holy Spirit, looked intently at him
Galations 1
11 But I make known to you, brethren, that the gospel which was preached by me is not according to man.
12 For I neither received it from man, nor was I taught it, but it came through the revelation of Jesus Christ.
13 For you have heard of my former conduct in Judaism, how I persecuted the church of God beyond measure and tried to destroy it.
14 And I advanced in Judaism beyond many of my contemporaries in my own nation, being more exceedingly zealous for the traditions of my fathers.
15 But when it pleased God, who separated me from my mother’s womb and called me through His grace,
16 to reveal His Son in me, that I might preach Him among the Gentiles, I did not immediately confer with flesh and blood,
17 nor did I go up to Jerusalem to those who were apostles before me; but I went to Arabia, and returned again to Damascus.
It also does not matter the sin, sin is all the same in God's eye no matter how "little" we think a certain sin is.
James 2:10 For whoever shall keep the whole law, and yet stumble in one point, he is guilty of all.
1 John 3:15 Whoever hates his brother is a murderer, and you know that no murderer has eternal life abiding in him.