These verses explain what it means to be merciful as God is merciful ( Luke 6:36). The first two examples are negative and the second two are positive. A judgmental attitude is not merciful. However some judging is necessary, so Jesus clarified that He meant condemning other people specifically. Judgment and condemnation are essentially God"s functions, not man"s. Rather a merciful person pardons others. Throughout the Sermon on the Mount Jesus was addressing interpersonal behavior, not the judicial system. Giving to others is also merciful behavior. What a person sows he or she will normally reap for evil or for good (cf. Galatians 6:7). Disciples will discover that they will receive back the same treatment that they have dispensed abundantly from God if not from man. [source][source][source]
"The saying here may appear to speak in terms of strict retribution, but the thought is rather that human generosity is rewarded with divine generosity, not with a precisely equivalent gift from God." [1][source]