Let's let Jesus explain it Himself:
21) Ye have heard that it was said by them of old time, Thou shalt not kill; and whosoever shall kill shall be in danger of the judgment:
22) But I say unto you, That whosoever is angry with his brother without a cause shall be in danger of the judgment: and whosoever shall say to his brother, Raca, shall be in danger of the council: but whosoever shall say, Thou fool, shall be in danger of hell fire.
That's what the One True God thinks about how people should treat one another. Never, did Jesus or any of the apostles advocate slaughtering people who disagree with Him. That's God's business, not ours.
So what changed?
Jesus Christ is the same yesterday, today and forever (Hebrews 13:8).
We don't kill people to please God. Killing people angers God. Instead, we follow more of Jesus's commands in the Sermon on the Mount:
43) Ye have heard that it hath been said, Thou shalt love thy neighbour, and hate thine enemy.
44) But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you;
45) That ye may be the children of your Father which is in heaven: for he maketh his sun to rise on the evil and on the good, and sendeth rain on the just and on the unjust.
46) For if ye love them which love you, what reward have ye? do not even the publicans the same?
47) And if ye salute your brethren only, what do ye more [than others]? do not even the publicans so?
48) Be ye therefore perfect, even as your Father which is in heaven is perfect.
That seems pretty self-explanatory. As Jesus would say, "Do this and live." (Luke 10:28)
Jesus Christ is the way, the truth and the life and nobody comes to the Father but through Him (John 14:6).
Follow Jesus and find eternal life. Reject Jesus Christ and you're rejecting eternal life.
Let me leave you with one last sobering piece of information:
7) He that overcometh shall inherit all things; and I will be his God, and he shall be my son.
8) But the fearful, and unbelieving, and the abominable, and murderers, and whoremongers, and sorcerers, and idolaters, and all liars, shall have their part in the lake which burneth with fire and brimstone: which is the second death.