David thanked God for vindicating him and for preventing him from doing evil. Abigail had been the instrument that God had used to do this (v. 39). It was proper for David to give thanks since he had left Nabal in the Lord's hands and had not sought revenge.
It is easy to see why David found Abigail so attractive. Not only was she intelligent (cf. 2 Chron. 30:22; Ps. 111:10; Prov. 13:15) and beautiful (v. 3; cf. Gen. 29:17; Esth. 2:7), but she was also a soul sister with David (cf. Jonathan). She shared his view of life and his commitment to God. However since from creation God's will has been monogamy (Gen. 2:24), it was wrong for him to marry her (v. 39). He had also previously married Ahinoam of Jezreel (v. 43). Perhaps he justified his second marriage with the fact that Saul had taken Michal from him (v. 44). Perhaps he got into polygamy also because it was customary in the ancient Near East for great warriors and monarchs to have many wives and concubines (mistresses). Yet God forbade this of Israel's kings (Deut. 17:17).
In any case David did not restraint himself in his relations with women, and this caused him major problems later in his life. The same words "sent"and "took her"appear both here (v. 40) and in the account of David's affair with Bathsheba (2 Sam. 11:4). We see here the seed problem that bore bitter fruit in David's adultery.
Should Abigail have agreed to become David's wife? It appears that she had a choice (v. 42). I do not believe she should have agreed to marry a man who was already married to someone else (Gen. 2:24), if she was truly free to decide. Abigail may have felt a need for security since her husband had died, and David was an attractive man with whom she shared much in common. Furthermore he was destined to become king. Yet he was married. Her decision is certainly understandable, though not commendable.