There is valid reason to reject the notion of HRWs impartiality.

Take for instance HRWs repeated allegations against the Syrian government. While it is wrong to ignore human rights violations of ANY war party the real question is why HRW chooses to attack and denounce the Syrian army in a time when the latter is fighting the „Islamic State“ and Al Qaeda (like Syrias Al Nusra Front).
Also, If HRW seriously claims to be concerned regarding Syrian lives it should encourage ANY peace talks that would help ending bloodshed. Instead HRW one-sidedly emphasized on the barrel bombs used by the Syrian army even using a picture of Kobanes destruction by the US airforce to highlight the devastation of rebel-held Aleppo by the Syrian airforce:
http://www.moonofalabama.org/2015/02/human-rights-watch-accuses-syria-of-barrel-bomb-damage-created-by-us-attacks.html

In general, HRW trivializes too much:
The Syrian army is – despite being portrayed otherwise by the international mainstream media – not the only party causing civilian deaths. According to the famous Aleppo blogger Edward Dark who criticizes both the government and the opposition the rebels shelling of Aleppo alone has killed in excess of 2800 civilians:
https://twitter.com/edwardedark/status/580044825426292736?s=09
The blogger even said in the end of February (2015) that „the rebels have killed more civilians than the regime“ in Aleppo:
https://twitter.com/edwardedark/status/571287191575506944

HRW was also quick (and totally wrong) in accusing the Syrian army of having used chemical weapons in the infamous Eastern Ghouta (Damascus) attacks of 2013. HRW, certainly no experts in missile technology and ballistics erroneously claimed that the rockets hitting different locations around Damascus were both fired from the same Syrian army mountain top. This claim was refuted:
http://whoghouta.blogspot.de/2013/11/the-conclusion.html

To understand that HRW IS indeed biased and pursuing an agenda it is good to know who HRWs „Executive Director“ Kenneth Roth is. Ken Roth is at the same time part of the leadership of the „Council on Foreign Relations“ (CFR), an American think tank that – despite using a subtle pattern – has been defending wars and offensive military action under the pretext of defending human rights, fighting terror or the much quoted „responsibility to protect“ (R2P):
http://www.cfr.org/staff/b654
http://www.cfr.org/syria/r2p-crisis-following-un-syria-vote/p27303
http://www.lobelog.com/cfr-liberals-again-pushing-for-another-middle-east-war/

While it is true that HRW occasionally criticizes the US and Israel, this is merely to keep up the appearance of neutrality as such criticism is not intended to have the slightest negative consequences for those countries in particular or any pro western Nato or EU country in general. Noone is willing or able to impose sanctions on Israel or the US, let alone wage any war or „humanitarian intervention“ against those countries. Policies such as „responsibility to protect“ (R2P) have no external expression in case of western or pro-western countries. They will for instance never be applied against Saudi Arabia, no matter how much death and damage Saudi Arabias totally unjustified offensive war against the people of Yemen will cause.