You may be familiar with the Sermon on the Mount, which is most prominently featured in the Gospel of Matthew (and alluded to in other Gospels) as the core of Jesus' moral teaching. The largest part of the Sermon on the Mount is the section called the "Six Antitheses" [Matt. 5:17–48] in which Jesus takes six portions of the Mosaic Law and calls on his followers to do otherwise (usually, more than the Law requires). He plainly indicates that they had misunderstood the Mosaic Law.
If you will take time to read this section of Matthew 5 (Link) you will see the contrast between His teaching and the false interpretations of the law found in Israelite history and probably stll held by most of Jesus' contemporaries.
He begins each of his "antitheses" with the preface "You have heard...", (שןמע אני), which in rabbinical practice is the formal rhetorical way to present one view and then introduce the opposite view as the only correct one. For example, 'You might deduce from this verse [Lev 19:18] that thou shalt love thy neighbor and hate thine enemy, but I say to you the only correct interpretation is, Love all men, even your enemies.'
Jesus' six antitheses are on:
As you can see, Jesus ---who is the ultimate and perfect Divine Revelation, expresssed in and by and through His own Person --- unmistakably states that the will of God is against murder (unjust killing), retaliation, and hatred. He calls, not only for abstaining from these practices, but for pro-actively going much further: ultimately, treating your enemy "as neighbor," and treating your neighbor "as yourself."
The Catholic Church does not teach pacifism, but makes a distinction between just and unjust killing: the just use of force must be limited and defensive, and can be employed, even lethally, only by those whose duty it is to defend the community against aggressors. Intentional killing of blameless persons is strictly forbidden.
How that works out in practice is still a very complicated question: but this indicates the assumptions that Christians must adopt.
Mrs. Don-o, what then would the accurate interpretation of 1Samuel 15:3 be, if it is not a command to slaughter infants?