Multi delete checkbox on button click does not work as required

I have a grdiview in which I have added the Multi-delete functionality. Please see the code for your reference:-

<script type="text/javascript">
    function ValidateAll() {
        var chkselectcount = 0;
        var gridview = document.getElementById('<%= grdTeacherProfile.ClientID %>');
        for (var i = 0; i < gridview.getElementsByTagName("input").length; i++) {
            var node = gridview.getElementsByTagName("input")[i];

            if (node != null && node.type == "checkbox" && node.checked) {
                chkselectcount = chkselectcount + 1;
            }
        }
        if (chkselectcount == 0) {
            alert("Please select atleast One CheckBox");
            return false;
        }
        else {
            ConfirmationBox();
        }
    }
    function ConfirmationBox() {
        var result = confirm("Are you sure, you want to delete the Users ?");
        if (result) {
            return true;
        }
        else {
            return false;
        }
    }
</script>

Also see the button html:-

<asp:Button ID="btnDelete" runat="server" CausesValidation="false" CssClass="btn btn-danger" Text="Delete" OnClick="btnDelete_Click" OnClientClick="javascript:return ValidateAll();" />

The issue is that, when I check the checkboxes and Click on delete button, it asks for confirmation. When I click on Cancel it still deletes the row from the Gridview as well as sql table.

What should I do for the proper working of this. ? Please suggest