"And the first one emerged ruddy...” (Beraishis 25:25)
Commenting on this Possuk that refers to Eisav’s birth, the Midrash (Bereishis Rabbah 63:8) teaches the following:
"‘Ruddy’ - Rabbi Abba bar Kahana said: ‘Like one who sheds blood.’ When Shmuel saw Dovid who was ruddy, as it says, 'And he sent and brought him, and he was ruddy,' (Shmuel I 16:12) he became alarmed (and was hesitant to anoint him king).
He said, 'This one too will shed blood like Eisav.' The Holy One Blessed be He said to him, ‘Eisav killed of his own accord, but this one (Dovid) will only kill by the authority of the Sanhedrin (i.e. when Halachically permitted).’"
The Midrash tells us that the ruddiness of Eisav and Dovid was a sign that their nature would lead them to shed blood. Therefore, Shmuel feared to anoint Dovid as king over Bnei Yisrael, lest Dovid be a shedder of blood like Eisav. Hashem told him that Dovid would not be like Eisav because Eisav killed of his own accord while Dovid would kill only by the authority of the Sanhedrin.
This requires further analysis. Why was Shmuel so concerned when he saw that Dovid was ruddy? Shmuel knew that Hashem chose Dovid to be king which obviously meant that he was the right person for the job. It is therefore quite evident that Dovid was on the highest of spiritual levels and exceptional in his piety. That being the case, what was Shmuel worried about? Dovid would overcome his nature and not transgress the severe sin of shedding innocent blood, wouldn’t he?
Apparently, Shmuel feared that the character trait of murderousness in Dovid would cloud his judgment and cause him to kill an innocent person inadvertently, believing mistakenly that the victim deserved death and that it was permissible to kill him. Although Dovid had reached the highest levels of fear of Hashem and had a strong desire to do what is just, nevertheless, Shmuel thought that his murderousness trait would overcome his desire not to sin and would lead him to see things incorrectly and shed innocent blood, like the wicked Eisav.
