Try:
Sub Demo()
Application.ScreenUpdating = False
Dim Tbl As Table, Rng As Range, i As Long
With ActiveDocument
For Each Tbl In Tables
With Tbl
Set Rng = .Cell(1, 2).Range
Rng.End = Rng.End - 1
If LCase(Rng.Text) = "input" Then
.Columns.Add
.Cell(1, 3).Range.Text = "OBSERVED BEHAVIOR"
For i = 2 To .Columns(3).Cells.Count
.Cell(i, 3).Range.Text = "SAT"
Next
End If
End With
Next
End With
Application.ScreenUpdating = True
End Sub
Note: Your descriptions are quite inconsistent. You refer to cells containing 'Input' and 'Observed Behavior', but then demonstrate them with 'INPUT' and 'OBSERVED BEHAVIOR'. These are obviously different.