Why Sarah was angry to Abraham about Hagar Why Sarah blamed Abraham when it was she who suggested to Abraham to take Hagar? I find Sarah's anger here to be unreasonable when it was her fault in the first place.
Many Biblical scholars believe that Sarah was upset that Hagar perceived herself more important for Abraham than Sarah. She wanted to become the "main" wife of Abraham.


  (Gen. 16:4) "... When she saw that she had conceived, her mistress was despised in her eyes."


Some commentaries from http://biblehub.com/genesis/16-4.htm:

Ellicott's Commentary for English Readers

"(4) Her mistress was despised.--Hagar, we are told in Genesis 16:3, was to be, not Abram's concubine, but his wife. She was to be Sarai's representative, and though now she would hold the highest place in the household next to Sarai, because of this relation to Abram, yet she would continue to be Sarai's maid. But no sooner had she conceived, than, proud of her superiority over her mistress, she wished to overthrow this arrangement, and, at all events, acted as if she was Abram's wife absolutely, and thrust Sarai aside."

Pulpit Commentary

"Verse 4. - And he went in unto Hagar. בּוא אֶלאּ, a linguistic peculiarity of the Jehovist, occurring Genesis 29:21, 30; Genesis 30:3, 4; Genesis 38:2, 9, 16 (Vaihinger, Davidson); but by some partitionists Genesis 29. and 30. are assigned to the Elohist (Tuch, Bleek, De Wette). And she conceived: and when she saw that she had conceived, her mistress was despised in her eyes. As Hannah by Peninnah (1 Samuel 1:6); barrenness among the Hebrews having been regarded as a dishonor and reproach (Genesis 19:31; Genesis 30:1, 23; Leviticus 20:20), and fecundity as a special mark of the Divine favor (Genesis 21:6; Genesis 24:60; Exodus 23:26; Deuteronomy 7:14). Whether Hagar imagined Sarai to be through her barrenness "tanquam a Divino promisso repudiatam" (Lyra), or anticipated Sarai's displacement from her position as Abram s wife (Inglis), she, immediately on perceiving her condition, became insolent (cf. Proverbs 30:23)."

Gill's Exposition of the Entire Bible

"And he went in unto Hagar, and she conceived,.... The formality of the marriage being over, he enjoyed her as his wife, and she immediately conceived by him: 

and when she saw that she had conceived; when she perceived that she was with child: 

her mistress was despised in her eyes; she thought herself above her, and treated her as her inferior, with contempt, and reproached her for her barrenness, as Peninnah did Hannah, 1 Samuel 1:6; and it was the more ungrateful, as it was at the motion of her mistress that she was given to Abram for wife."

Matthew Henry Commentary

"16:4-6 Abram's unhappy marriage to Hagar very soon made a great deal of mischief. We may thank ourselves for the guilt and grief that follow us, when we go out of the way of our duty. See it in this case, Passionate people often quarrel with others, for things of which they themselves must bear the blame. Sarai had given her maid to Abram, yet she cries out, My wrong be upon thee. That is never said wisely, which pride and anger put into our mouths. Those are not always in the right, who are most loud and forward in appealing to God: such rash and bold imprecations commonly speak guilt and a bad cause. Hagar forgot that she herself had first given the provocation, by despising her mistress. Those that suffer for their faults, ought to bear it patiently, 1Pe 2:20."