They say victims include those seen as being opposed to the terror group’s rule. 

【翻译】他们说，受害者包括那些被视为反对恐怖组织统治的人。

为了理解方便，我把省略的成分补充完整：

They say victims include those people who are seen as being opposed to the terror group’s rule. 

be seen as：“被视为；被认为是”。as 是介词，后接动词时须用动名词。

be opposed to：“反对”。

被视为反对：be seen as being opposed to...

as 经常跟“观点”类动词连用，表示“作为......，是......，以......方式”，又如：

I saw the job as being very tough.

我认为这活儿很棘手。

The job was seen as being very tough. （跟你的句子一样）

这活儿被认为很棘手。

You mustn't think of me as being unhappy. 

你不要以为我不高兴。

The astronaut accepts danger as being part of the job. 

太空人承认他们的工作中包含著危险。

He strikes one as being somewhat lacking in humour. 

他使人感到有些缺乏幽默感。

She struck me as being very practical. 

她使我觉得她很实际。

I count him as being very reliable. 

我以为他是很值得信赖的。